Of course people know the predefined opening for the specific map gen. These are map gen maps. They are not complicated maps with specific reclaim timings anywhere near some premade maps. You can just watch tons of 1800+ or 2000+ map gen games and see people maintain parity in macro because they are aware of the correct moves to macro properly (or too bad to notice the 3000 rated way to macro better). If you can’t keep up in macro you lose the game, that’s what everyone teaches new players and it’s why they first learn how to get to 3 factories without stalling when being trained. If you can’t scale up and keep pace, even 10k mass in selens will kill you. Apply that to map gen and anybody that can’t keep up in macro will lose rating, but it just doesn’t happen.
The BO stuff you're talking about is exactly what is still present in map gen. Pros do things second nature because they have thousands of hours and can just know what a terrain/mex setup will do for the game state. Combine that with knowledge of how games work in low player and high player counts. That means they pick the correct start whether it's all in t1 spam, super early t2 mex, quick drop, quick ACU move, or what. Nobody has 30 15 minute BOs in their brain, they just know the way a map is going to play. Or they know what they need to do a specific play because they’ve done it before on maps whether in term of game state or eco.
The extent of BO that exists in ladder, or frankly just about any teamgame that isn't dual gap or senton (not that these BOs are insanely difficult or essential to know to be relevant even in a 2k+ lobby) is basically that far in term of "set up knowledge." If you account this as BO, then map gen has the equivalent level of BOs. If you don't, then neither map gen nor essentially any ladder game has BOs.
It just comes down to whether you think learning the game's mechanics is just another way of saying BO (where both map types have these BOs, because it's still the same game) or if it doesn't count as BO because it's impulsive moves in the moment (where again, both map styles have it since nobody is tryharding the shit out of ladder). The times where a difference exist is essentially tournaments farming out sandboxed BOs because of a map pool release 2 weeks prior to the event and it having the money to warrant the pain of doing it.
Just ask yourself if anybody is going 1st drone or some other totally worthless opening on map gen. No? That’s already “bo knowledge” and as you get higher in rating, the more false choices you are able to recognize and avoid. I do not define that part of the game as BO.