I agree.
It should have been presented for what it is, or likely is. Not that many 1777 pattern muskets made it over here during the war, and considering that Regiment was largely back in the UK getting rebuilt from '78-'83, it just raises questions. A turnscrew and read of Goldstein & Mowbray's book would be enough to look for some clues.
I don't consider Ian a run of the mill youtube pseudo-historian. He's built a niche market as a knowledgable presenter of historical and unusual firearms, because he actually is pretty good. Good enough to parse secondary and even primary source material - at least in the areas he knows.