Why would a thief register his stolen bike online after the fact? I think you've got this mixed up, these registries only work when people consult them before buying.
I don't think he ever implied that a thief in particular would register the stolen bike. The first scenario is a buyer checking beforehand to see if it was stolen. The second scenario is the owner of a stolen bike checking to see if someone else has re-registered the bike, like someone who purchased the stolen bike thinking it was a legitimate sale (a bit misguided, as the system doesn't quite work that way).