I’ve checked the forums, and it seems this has been a problem for some time. It was the only time I’ve seen it personally. I already did an in-game bug report but wanted to bring it up here.
We have video proof from myself and another person who saw it happen at a minimum. I was even lucky enough to see the siege change from invisible (and untargetable) to visible when the game ended, to witness the person doing it hopping out. It was especially interesting because it was able to get inside the Alliance keep before the walls were even down, and easily travel inside and outside of the walls at will.
They also bugged the Hangar flag but I’ll do another post about that.
They had 2.5 billion damage and no deaths. Their guild name indicates they will not be banned though.