Say there was a mechanic that says you can't wear plate armour because your strength ability score is too low. Isn't that telling people how to play their character, just as much as it would be to mechanically define the meaning of "resting"?
No.
Saying you need to be a certain strength, or combination of strength and bulk, in order to effectively wear plate armour is perfectly realistic. It makes sense.
But telling players that their PCs aren't allowed to rest (or that DMs are instructed to disturb that rest until some peg-point is reached) when logic and self-preservation would say that they should isn't realistic at all, and makes no sense other than from a perspective of pure small-g gamism.
The objection seems to say there a mechanical rather than narrative method for managing rests should be preferred, because a narrative method risks telling people how to play their character whereas a mechanical method will be no different from the mechanic for their AC on wearing certain armour. It's a constraint. Constraints are what make games.
Other way around. My objection is that mechanical management of rests forces undue and unwarranted constraints on to the players and-or the DM; where narrative management - with the players in this case controlling the narrative and making the decisions - has no such problems.
I agree that constraints make games, but those restraints have to make sense in the context of what the game is trying to achieve.
A chess rook can only move in straight lines, while a bishop can only go diagonal. These are constraints that make perfect sense in context, that context being the game is trying to force the players to think their moves out within those constraints.
But D&D isn't just trying to achieve this, it's also trying to achieve a state where players and the DM between them control what happens in the fiction. Mechanical resting constraints fight this control
Another example of the same thing would be the game forcing a particular method of in-party treasury division, instead of leaving it up to each individual party/table to determine its method for itself.