I think that comes from a misunderstanding that the AK is inexpensive (because they made so many of them) and how many experts mention it's loose tolerances so it must be simple to make.Makes more sense than that one where an aeronautics engineer who explicitly stated he can't make complicated things ending up making a goddamn Klashnikov
That said an aeronautics engineer being able to make firearms is plausible, both industries have similar tolerances. He could definitely make a percussion cap musket with the tools of the era. He might be able to make a normal double barrel shotgun.