This is a distant cousin to my 50% serious question about whether thumb placement matters on throwing a fastball. Like, some dudes curl their thumbs and others don't. I could never throw with a curled thumb, and it can drive me bonkers trying to figure out if it matters.
my dumbest baseball opinion that I don't really believe but still maybe kind of believe: i just don't believe where you grip a fastball has that much impact on how it moves. they must be lying to us, all seams are the same, they're doing something else to it