How do things change if every single person in the musical ensemble had absolute pitch? How would it be better, and how would it be worse?
What the post you're replying to is alluding to is all the ways that pitch is simply not absolute. That can be for boring reasons: you're playing with an organ, or some other instrument with fixed or temperature dependent pitch, or in a different city with different conventions; but also for the interesting reason that the "correct" pitch of a C, say, is the result of a mostly-unconscious negotiation that considers ones' place within a chord that's sounding and the melody you're playing. Even equal-tempered tuned keyboard instruments involves compromise wrt placement of the octaves, etc.