I don't think it's inconceivable that the competition was really close all throughout the preseason. However, there was always the possibility that if Mac impressed while Cam did not, the team would not hesitate to name Mac the starter. As for that 3rd preseason game, there was no point cutting anyone prior to that; far better to keep the roster flexibility in the event of an unexpected injury. If Mac got hurt in PS3, then Cam would have been named the starter. Easy decision to make when you are allocated 80 roster spots.
Mac got lots of joint practice time with the starters when Cam was forced to miss practice, and will have plenty of reps with the starters the next 10 days. I do expect the offensive game plan to be relatively simple for Week 1, however. But that would have been the case no matter when he played against the Giants.
As for Belichick, he has to say that Cam's vaccination status had nothing to do with his release if he wants to avoid a grievance being filed against the team. He didn't have to say the part about vaccinated players getting sick as often as unvaccinated; his statement could create an unwanted and frankly unnecessary distraction. Still, Bill does get a mulligan every now and then; he's earned them.