In this case, the action points directly and doubtlessly to the motivation.
Well, if you want to assume that then yeah I can see why you have the view you do. I mean, it's false, and I can tell you that with certainty because I know what goes on in my brain and you don't. But if you want to assume you do know what goes on in other people's brains, and you want to assume that it's nothing other than to gain advantage, then I can see how the behavior would bother you.
And in effect giving your character an extra skill/ability along the lines of "Knowledge: Monsters" for free.
That depends on what you think the benefit of Knowledge: Monsters is. Just because I
believe my conception of monster knowledge is accurate does not mean that it is. If the DM changes a monster, or introduces a new one, it does not necessarily follow that what I have decided my character thinks is accurate. If I decide to not trust my knowledge, and ask the DM if my character knows something, that's where the Knowledge: Monsters would come in.
I can decide my character believes he can fly, right? That doesn't mean that when I declare I jump from a cliff I actually fly.
Which means if I, as another player, have put a feat or skill points or any other char-gen resource into giving my character that same ability I'd have every right to feel a bit pissed off.
Why? That other player can actually ask the DM for information, and the DM may allow that person to roll with proficiency.
In my view the bolded is and always has been the baseline default, not the exception.
Yes, I understand that. I'm just pointing out that it's not in the game; it's your preference. Even if you take as gospel what Mentzer wrote in 1983 about character knowlege vs. player knowledge, he doesn't define where that line is. He may have simply been referring to the version of "metagaming" mentioned in 5e, where you think, "Well, I don't think the DM would give us this challenge unless...etc." That is also an example of using player knowledge rather than character knowledge.
But YOU have decided what in means for your games, which is great. I'm glad you like playing that way. But by no means is that an intrinsic part of the game. It's just the house rule that you (and presumably others) like.