It seems to again come down to the possibility of a rapid and unexpected jump in capabilities.

We could test it in thought experiment.

Chess game human-grandmaster against AI.

1. it is not rapid (not checkmate in begining).
   We could also suppose one move per year to slow it down. It bring to AI next advantage because it's ability to concentrate so long time.

2. capabilities
   a) intellectual capabilities we could suppose at same level during the game (if it is played in one day, otherwise we have to think Moore's law)
   b) human lose (step by step) positional and material capabilities during the game. And it is expected

Could we still talk about decisive advantage if it is not rapid and not unexpected? I think so. At least if we won't break the rules.