By some rule that I don't understand, he has to quit at some point so Harry can call for a vote.
He isn’t technically filibustering.
That’s because the bill had not yet been officially introduced, or something like that. Because it’s technically not a filibuster on a bill, the rules make him stop when Reid takes the floor to call for a vote to proceed on the bill. BTW, this vote is not the cloture vote to shut down debate on the bill. It is preliminary to that. It’s just a vote to start on this bill.
Roughly, this is why Reid can stop him and he can’t continue when Reid does.