I've tried it with other characters and it does work and will pick them up off the floor, but there's a catch, they can block.
I have a theory as to why it works with Geras. It's because Geras' 122 knockdown has an optional followup, and because of that optional followup I'm assuming that the opponent is still left in a combo state with their blocking disabled.
I tried finding other characters who had similar situations but couldn't find any. Although I don't know every characters strings. It's possible a few characters have the same ability