It's tricky in AD&D, which does not have a very robust action resolution system for anything outside of combat and dealing with doors.
Actually it is decently robust, since the only actions that required resolution were the things associated with dungeons and adventuring.
But sticking to AD&D - suppose that, down the track, the players fail at something - eg they fail to prevent an escape (by a Kobold, or a known spy, or whatever). Then that could be a trigger for the GM changing things in the backstory to "undo" the quality of the intelligence the players have gathered.
Why would you need to fail before things could change? Things change all the time without the party needing to directly impact them. Perhaps the DM has a timeline for the events independent of the players? (FWIW, the story goes on if the PCs aren't involved in most games, I do this all the time).
If you had done (A) instead of taking the time to capture and interrogate the kobold, the party would have been at location (B) earlier, preventing (C) from occuring. Because (C) occured in the timeline while you got information from the kobolod, that information was no longer valid--plans had moved forward in your absence.
Or suppose the players interrogate a NPC, and it passes a morale/reaction roll, then maybe the GM has it feed the players/PCs false information.
How do you know this didn't happen? I roll dice all the time in my games without the players realizing I've rolled anything. Especially then when "rolling in the open" wasn't common practice?
But just arbitrarily feeding the players false information, even though they have successfully captured and interrogated a NPC, in my view is a railroader's technique. The point of it is to keep the players floundering, without the information necessary to make meaningful action declarations. It keeps the GM in control of the unfolding of events.
Here's the thing, it's entirely possible the kobold captured doesn't know any useful information. In the military, information is need-to-know and infantry could easily not know the larger picture or even the immediate plan.
Why do you (party) get to decide that just because you succeeded in capturing the kobold and intimidating it into talking it must know meaningful intel??
You dont' decide that, the DM decides what information, if any, the kobold would have. This is something about the game world that is beyond the PC and is the pervue of the DM.
It is like me driving to the store to get milk. I expect once I successfully turn on my car and drive to the store, finding it still there, I should be able to buy milk. But what if I get there and there is no milk? I dont'get to decide that there MUST be milk.
Like I've said, I understand the players' disappointment that after planning, executing, etc. a successful plan, it netted nothing useful. But that just doesn't constitute a terrible DM for me.
Now, if everything else the DM did was ok, at this point the group could have said "Well, that didn't work... so, plan B?" or something. I know you've said you felt it was a waste of time, but if you had fun in the planning and capture, it doesn't seem like a waste of time IMO just because the result wasn't what you expected.
Not having information doesn't create the same "fog" around player action declarations for their PCs as being given false information (but presenting it as true).
True, you have nothing to act on instead of something wrong to act on. Both are "fogs" just the same. Using the milk buying analogy above:
Suppose I am talking to my friend who tells me they were at that store and bought milk yesterday. They know it was there. So, I hop in my car and drive there, only to find out it's all gone.
Or I never had that conversation but I still go to the store a few days later and find no milk.
Either way, in the first I have false information, in the second I have none, but in the timeline of events, at some point between my friend buying their milk and me finding none, the milk disappeared. The store might have pulled the remaining stock for some reason, or perhaps there as never a lot to begin with and the remaining milk was bought before I got there?
Regardless, both "fogs". Yes in the first I choose to act because of the misinformation, but in the second I act of my own initiative. Result is the same: I am disappointed I have no milk.
And as I posted not far upthread, having a PC collapse from a heart attack or stroke, or having a red dragon fly overhead an burn them to a crisp, is also quite plausible. The test of good GMing is not can we imagine something that makes sense of the fiction the GM presents - that's a threshold that even the worst GM imaginable, and the biggest railroader of all time, can step over.
This isn't the same thing at all, however. Killing of a PC in the narrative is hardly the same thing as informing the party they failed to do or gain something. Such things would be terrible DMing IMO, because it takes away
all agency from the player.
Now, I can see a scenario with the dragon being more plausible, but even then the goal is to make the game challenging for players, not put them in a no-win situation where escape is impossible.
To be clear, I am all for unwinnable encounters, but then escape or capture must be viable options to the PCs. Just killing them off due to overwhelming foes without any avenue of survival (even temporary...) is a no-no.
Yet five people acted as we did. So either, as per
@Lanefan's suggestion upthread, we were weird conspirators setting out to waste our own time and the GM's; or we had the most rarefied tastes of RPGers ever; or the game was actually terrible!
Maybe the game was terrible for other conflating reasons? You've hinted as such IIRC. But the main point of contention about how the DM played an NPC kobold captive is very much insufficient for myself and others.
I understand the result was disappointing (no milk

) but that alone doesn't make them a terrible DM. That would be like me deciding I will not use that store anymore because there was no milk that time.
Now other factors might contribute to that decision. They are frequently out of milk. The prices in general are high. And so on. Such factors might compel me to go to a different store where I might have to drive further, but I'll get what I am after more reliably.
I can very easily imagine this being the case in your situtation. One thing the DM does rubs the wrong way. You let it slide because in general you're having fun. Then soemthing else happens--"If I was DMing that isn't how I would have done it". But hey, it isn't your game to DM so again you let it go. Finally, the kobold incident and you've just decided enough is enough... your fun is materially being affected and it is time to move on. In between sessions the players might discuss the first incident, and then the second, and finally the last during the game brings you all to a concensus