What they're probably doing is simplifying 'Romano-British' into just Romans. Some of the medieval sources we have that treat Arthur as a real guy (he probably wasn't, for the record) specifically place him in the late 400s-early 500s. Historia Brittonum, specifically.
While Rome had abandoned the island at that point, the syncretic Romano-British culture still survived, and it is they that were subject to the Anglo-Saxon invasion.

This isn't shocking at all. TrueAnon has always just been Leftist Themed Joe Rogan. Of course their podcast encourages shit like this.