He touched him because he was pushed into him. That by definition is not goaltender interference. The contact has to be self-initiated and it has to impede the goalie from making the save. Neither of those criteria are definitively established here so calling it goaltender interference at all, let alone overturning the call on the ice of a good goal, is ridiculous.
As you said, nobody knows what goaltender interference is which is a big problem, but this sure isn’t it. It was potentially a game and tournament changing call to boot, and they overturned the call on the ice without clear evidence that the officials made an error. Pretty tough.