If the person who made that video, correctly focused the camera, the comparison would be a little bit more fair.
I can get video out of my HX9V ($350 Camera) that looks as good or better than that video.
The 5D has a vastly different lens than a cell phone, part of it, is the ability to focus to a certain depth, rather than having the entire shot in focus.
But yes, the OnePlus does take great video, but I hope people only take this as a comparison, and not base the "quality" off of it.
The 5D is still better as a camera and a video camera, no way around it.