You're not wrong in questioning a claim that comes off as incorrect. The Great War channel is unfortunately not as accurate as it claims and most of the research it does is highly outdated and plain out wrong.

Timothy Winegard, one of the foremost researchers of the experience of the First Nations (and other Indigenous peoples) during WWI, writes in his Indigenous Peoples of the British Dominions and the First World War (Cambridge University Press, 2011) that Pegahmagabow's claim is just that: a claim. We have no real corroboration beyond Pegahmagabow's own claim that he managed to get 378 kills. Like you mention, it's difficult to get kills confirmed and even more so if you, like Pegahmagabow, worked alone. Like many other claims in modern conflicts, his claim became repeated as fact and from thereon accepted. This shouldn't in any way retract from the fact that Pegahmagabow experienced an astounding military career with all the honors that came with it. What we do know is that the claim came directly from Pegahmagabow himself (so not something invented by propaganda), but it still remains to be confirmed by other, reliable contemporary sources.