It doesn't really matter at what hasrate they fork. BTU will still be an alt and Polo said they might not even list it. And they can fork at at any %. They don't need to wait until 51% or 76% or any other number.
without the right consensus they can not fork, otherwise they would end with an altcoin at 100%, after all all the altcoin are hard fork of bitcoin without reaching any consensus, this is the point of consensus to have the original chain upgraded and not a new one