He can't shift his weight for a shot across his body before the ball is hit, because he doesn't know if Ayew is going to intentionally try for front post?
He doesn't have inhuman reaction speeds. He'll have decided it was cross goal probably around the same time it deflects off Spence, hence the movement just after.
This isn't a dig or a "if you don't have experience your opinion doesn't count" type thing, but have you played goalie at any level? Because if you have, you'd know just how difficult a deflection like that is to react to.
I'm not sure the change stems from Warnock though. I think with Warnock it's a boy that cried wolf situation.
The fact that Lineker and other prominent members of the game have also questioned the disallowed goal, is when I think the FA will stand up and take notice.