I also don't think that you can really use simulationism to justify events outside of the player's meaningful control that lead to a bad play session.
Nothing of the described situation makes this applicable. What happened was completely in the hands of the players -- they had absolutely meaningful control.
If I have incredibly good simulationist reasons for having a dangerous high level dragon in a particular region, but the players don't know enough about the situation not to blunder into the dragon and get their characters killed before they can retreat, that's still my fault. Their actions led them there, but their actions were made based on the information available to them. And assuming that my players didn't actually WANT to get killed by a dragon, that suggests that I didn't give them the information they needed.
Right, but your failure is not being very good at simulation and/or verisimilitude, not placing the dragon. A massive, ancient predator like a dragon has an impact on a region. It's presence isn't going to go unnoticed. And if the players "blunder" into the dragon -- meaning, most likely, that they wandered off in the general direction of the dragon without bothering to find out how powerful it was, just assuming it must be a Level Appropriate encounter because that's how the game "works" then it is the players' fault. On the upside, they got to make a meaningful choice.
And the question of whether the players didn't "want" their characters to get killed by a dragon is sort of irrelevent. Did they want to go on an adventure, in an area in which a dragon might be found? Did they want to have the opportunity to gain treasure, XP and fame? If so (and the answer would seem to be "yes" given they are playing D&D) then they also know that there is *always* a possibility of "blundering" into an encounter with a deadly dragon (or liche, or demon or whatever).
Let's take the example of the good old fashioned Random Encounter Chart, something that seems to have fallen out of favor in the modern era. A random encounter chart (for, say, a wood haunted by dark fae) serves both a play purpose and a setting purpose. It provides potential interesting things during an otherwise rote activity (travel and/or exploration), as well as describing the micro-cosm of the fae haunted wood. The DM need not spend thousands of words describing the wood (though he still can) if he has a well developed random encounter chart for the place. In addition, the random encounter chart for the wood provides information to the players -- assuming the bother to ask or look. While the DM might not give the players the chart to read, he can (and should) relate its general contents and various probabilities through NPCs to inquiring players.
For example, the PCs arrive at a little town on the edge of the Wytchwood and need to follow the road through the wood to get to their next destination. While having a few ales in the inn that night, the locals regale them with "ghost stories" about the wood and warn them to stick to the path, etc... Intrigued, the PCs ask why. At this point, the DM looks at the encounter chart for the wood --
likelihood of encounter:
Day one road: 1 in 12 per 4 hours
Night on road: 1 in 6 per 4 hours
Day off road: 1 in 6 per 2 hours
Night off road: 1 in 4 per hour
roll 1d12
1-4 Goblins (2d4+2)
5-7 Forest Gnomes (1d4+1)
8-9 Pixies (evil (1d4)
10 Treant (evil)
11 Dryad (good)
12 Green dragon (roll 1d4 for age category)
-- and has an NPC, for the price of an ale, tell how goblins haunt the woods, hunting the poor gnomes, but the real dangers are the unseelie fey and angry trees, but there's hear-tell of a helpful forest spirit. And there's this story of a green dragon laying her clutch of eggs in the forst some genrations back... Travelers on the road don't usually have much trouble during the day, but those who venture into the forest day or night sometimes never return... Etc.
Now the PCs have information. With this information they can do whatever they like. They might do the "sensible" thing and wait till morning and make a hard push to get through the forest by nightfall. They might go in straight away, hoping to kill themselves a dragon. That choice is the players' and whether the woods constitutes a "Chekov's Gun" or not doesn't matter at all. The DM's job is to adjudicate the choices the PCs make and answer the questions they ask.
If the PCs have more pressing issues at hand but decide to go dragon hunting (or looking to help the gnomes, or whatever) instead, that's there business. if they do so because they "assume" everything is an adventure hook, they have been playing with railroady DMs for far too long and should get used to the wonderful feeling freedom and stretch their wings a bit.