I don't think he's using genius to mean "a genius". I think it's instead meant as a term for "a bright idea". i.e.: "to have genius" vs "to be a genius".
I am curious about how you come by your view. Have a look at this defense [0] of Hamilton on the $10. I can see from it how many could see him as a genius.
He is saying he has been credited with genius, that is intellectual power or talent, not that he has been credited with being "a genius". The former is now an archaic way of using genius, the latter was pretty new in Hamilton's time.