I don’t have to watch that video to see how ridiculous that sounds. Imam Mahdi will come to this earth to unite all Muslims. There might be situations where Sunnis and Shias will claim him but that will be quickly wiped out by Imam Mahdi himself.
As far as the Shia biased is concerned. I am a Sunni and have no problem with their POV. As a matter of fact they even praised the first 3 Caliphs in their documentary. They might not even be Shia after that episode.
They made it clear how Muslims are not the enemy of one another. We shouldn’t waste our time bickering over historical times. And instead work on our Iman and prepare for the final day.