After the catch he is a runner and no longer a receiver.
I feel that is the least important part of this rule change.
A88 jumps in the air to catch a pass, and secures the ball in his hands while still airborne. B56 launches himself shoulder-first at A88 and hits him shoulder to back a half second after A88's foot touches the ground in bounds.
Is this not a hit on a defenseless receiver because A88 technically completed the catch and is therefore no longer a receiver?
Of course not, that's absurd. The rule states that the player is defenseless if he has "not had time to clearly become a runner". He can absolutely complete the catch, become a runner momentarily and still be a defenseless receiver -- and for safety purposes, I'm erring on the side of caution here and, if he doesn't lose possession of the ball after the hit, I'd be inclined to enforce from the end of the (albeit very brief) run.
I do see this starting to get into the weeds of the NFL definition of "an act common to the game" or whatever-the-devil language they're using these days... and when that starts up, I'mma head out.