I don't think it's just a case of being cynical. if you think about it, people are most likely finding that series because of searching specifically for warhammer related stuff and finding that as part of their search, not searching directly for that series itself. games workshop has access to money and professional marketing and all kinds of resources the average person doesn't have, so it only makes sense their "brand" would be more well known.
the smart thing for that guy to do now would be to start a new series with original characters, so he can piggyback off his past success while also branching off from using a copyright he has no control over. he has enough eyeballs on him already now to potentially go independent. I doubt he'd have as many viewers as he does doing warhammer stuff, but that's still better than being unable to make videos at all!