If you want to stick to the rules, then fine : Usyk's belly button wasn't visible when he took the punch.
Also, if Dubois was the champion fighting in UK and Usyk the contender, we all know what would have happened. So if you prefer, Usyk was lucky because some referees could have taken another decision, especially in the scenario i presented.
Also, whatever you think, Usyk looked bad (and Dubois too for that matter) : when was the last time you see a champion spending so much minutes on the ground in pain ?