The argument that "Google still controls Chromium so it's not good enough" is exactly the kind of FUD I'd expect to back up this kind of psyop, too.
I find this notion completely baffling. I use Chrome, Firefox and Safari more or less daily cause I test in all 3, and other than Safari feeling clunkier and in general less power-user friendly, I can barely tell the difference between the 3, especially between chrome and FF (well, other than uBlock working better in FF anyways).
It isn't unique to youtube either. Gmail offline mode only works on chrome, even though other browsers have the necessary APIs. And menu copy and paste in google docs uses a special chrome-only extension that google pre-installs in chrome, instead of the clipboard API that works in other browsers as well.