I think this truism is manifested most commonly in the oft-repeated idea that in an RPG, you can be/do anything. No, you can't; the entire point of having rules is to delineate what you can't be and/or do.
Id push back against that myself; I think theres a recurring theme of these games denying too many possibilities to players when they shouldn't be, particularly when it comes to emergent gameplay.
Things like the Peasant Cannon are obviously undesirable, and fixed relatively easily, but theres a lot of things that emerge in the same way the Peasant Cannon did that
should be readily allowed to occur, even if theres no rules for them.
This requires prepping GMs to be able to respond to these emergences and lean into them.
In video games, these kinds of things actually tend to express as literal bugs, and in the best of circumstances they get turned into features rather than patched out. For example, Rocket jumping in older arena style FPS games, where you can leverage the ability of a rocket explosion to propel yourself if you time a jump and aim right.
Not intended, but it does make for a fun time and opens up a lot of ways to play that wouldn't be there if the developers didn't lean into them (or at minimum merely permitted them).
In TTRPGs, with a far wider possibility space for interactions (that only gets wider the more robust the system is), GMs need to be able to handle these emergences and know when its truly appropriate to allow for them to play out, and indeed, become a reliable emergent feature the players can count on.
The Peasant cannon for example would only really be fine only in the most Gonzo of games; in most others itd be wise to deny it, on the basis of what I might call the Smart-Ass rule: Don't be one.
But even in DND, by the rules, the Cannon is a lot of work just to get 1d4 bludgeoning damage.
But, the Peasant Cannon is also an example of a design level problem with how the turn system was structured; while emergent, it was predictable as its existence is directly predicated on a very simple interaction between two game mechanics, Ready Action and the Turn mechanics.
Its easily fixed by simply putting in a byline that states a maximum for how many Actions can occur simultaneously, relative to the amount of entities in a given playspace, and what occurs when this limit is exceeded.
Ie, lining up 1000 peasants to do this is going to mean when the Actions trigger, its going to take the full amount of time each of those actions would have taken.
(its also fixed by recognizing the game isn't modeling real physics and the rules physics don't confer any speed to that rock when it passes from one person to another, nor does it provide any damage increase for such speeds)