It would be dumping if they took much less than 55%, but they actually do make profits so its not dumping.

YouTube has been in the red at least until 2010 under most estimations.

For reference that's around the point Vimeo started pivoting to different strategies and blocking long content as they couldn't pay for the infra.

That's also around that time that Dailymotion went down the pipes with the French gov stepping in to save the remains.

YouTube thrived from there as creators and advertisers had nowhere else to go at that point. That's the dumping part.