Alright, thanks for everyone for responding.
If its not clear why I brought this up, I'll unpack it (because its salient to the lead post).
Moldvay Basic was my introduction into the hobby in 1984 (I quickly assimilated a ton of other texts thereafter, but this was the first). Rather than actually playing, I GMed a game for my cousins (who were 4 years older than me and vets of the game by 3 years or so) at 7 years old using the text's principles, rules, procedures. It was very elegant, easy to follow, paint-by-number-ey.
It took me awhile to understand the brilliant engineering that went into this tome (the decision-point pressure leading from the Turn structure interfacing with the Wandering Monster and Rest Clock, etc), but one thing that stood out to me right away was "Monster Reaction."
In my gaming career, THIS was the first introduction I had to the "play to find out what happens" ethos. Before and by 7, your make-believe is completely unstructured. But here, here is a piece of structure that (a) dictates how the fiction emerges (constraining the GM) while (b) challenging the GM to imagine, within the confines of the dungeon's Theme/Scenario and Setting, how/why this creature is reacting the way it is and then frame the situation/challenge accordingly.
The dungeon Theme/Scenario/Setting is Remove a Curse (someone in the village stole an idol from a sunken temple and it needs to be brought back and replaced in its sanctuary). The dungeon is stocked with undead. Wandering Monster roll hits when the PCs double back to the vestibule to find another way in after they've explored the sunken entrance hall and found a swim is necessary to advance. I get 3 Lizard Men (who are Neutral but are not above capturing and bringing humanoids back to their tribes for a feast) and I, crazy enough, I roll a pair of 6s on Monster Reaction; Enthusiastic Friendship!
What could that possibly mean? Why would 3 Lizard Men be in the vestibule of a sunken temple and enthusiastically entreat the PCs toward alliance? What a creative challenge! This could go any number of ways, but I have to think on my feet, give life to this situation and frame it with vigor. Who knows what the PCs will do or what happens next?
I hope its clear how this looks just like the lead post's question and the games that we've been talking about above (except in those games, discoveries/challenges like this should hook into one or more of the PCs' thematic portfolios).