Eight*-passenger :doh: my bad hahaha.
But yeah it's not so much that Biff steals it, it's that he flies it like an expert pilot. Also, since they erased the timeline established in BttF Part II where Biff stole the time machine, it IS a stretch that Biff would not only take it, but have any motivation to. If Doc and Marty really had been successful in Part II, Biff would have no knowledge of Doc or the DeLorean or anything.
I mean, I know I'm being super picky but it's fun to be nerdy sometimes

BttF was my favorite ride at Universal while it was there and it was a great attraction--just super easy to pick apart if you're a geek like me.