I'm not entirely sure he does though. A QB holding a ball instead of a 2.4 second release could be because of factors not in a QBs control. Play call, excellent coverage, longer routes. I think a QB holding the ball based on those and then avoiding a sack is valuable. Limited value given what happens after that, but valuable.Etc covers a lot of possibilities.
In the thread someone asks him about QBs who hold the ball creating pressure, that's the part he handwaves.
And no... Evading sacks is not by itself good. Especially because he's not REALLY showing that he's calculating actual sack avoidance, just differential between pressure and sacks, which is doesn't necessarily mean much because not all pressure is as likely to become a sack (early vs late, in pocket or out, etc.)
I think evading sacks is good as it, by itself, means a much less likelihood of a negative play (ignoring Macs results). I think that differentlial you mention is likely a positive attribute given that Mac has taken sacks when he steps up in the pocket AND avoided sacks when he steps out of the pocket. Again, ignore Macs results as we know he needs to avoid the sack and not do dumb shit like fumble on his own 10 or throw across the field for a pick 6. Heck, the Saints pick 6 was Mac stepping up in the pocket and throwing while getting hit. Damned is he does, damned if he doesn't.