Law Enforcement use of force training has been teaching about this for 15 years or more and it’s in the MPD manual.
So Chauvin is presumed to know that prone restraint is dangerous. One of the officers actually is heard asking about repositioning because of ExD and is brushed off.
It’s not hindsight to recognize failure to follow training protocol.
The officer was brushed off because he was suggesting putting him on his side, which would offer no improvement on the positional asphyxia issue and would allow him to start kicking again. Face up is the dangerous position for resisting individuals.