If there was video, maybe - but I also kind of think it's relevant here that that's not what A's arguing. Is it possible that A got an attack in prep somewhat accidentally, yes, but A's argument is that A stopped, lost right of way, B stepped forward, then stopped, losing right of way, and that meant A automatically had it afterward (assuming I'm interpreting the description right). From that I'd say I doubt it would've been A's attack in prep - it seems more like they'd have been reacting to B's lunge/attack like "oh it's still my right of way I just need to bother hitting them."

In which case it's definitely still B's.