Do you mean you looked the profile up here in the forum? That really won't mean much, even I have a different in game name. If it's in game it's obviously an existing account. 

Good god these opressed bronze players are just reaching now lol

I figure that from the moment you load a new screen of opponents, all those matches are locked in for you no matter what. If they change their defense team (changing champs, changing gear, leveling up,...), it doesn't affect you as you would still attack their old defense team like it was at the time you loaded the screen. It sort of makes sense that this would hold even if they delete the account entirely. After all, I've never experienced that I tried to attack someone in arena and got an error saying that the account no longer existed, or anything like that.