That's like saying an AR is a "new" rifle/invention because it's chambered in a different caliber or has a different barrel length, isn't it? The AR-15, M16, SP1, M16 A1, M16 A2, M16 A4, M4, AR-10, etc... may have all had minor variations from the original but they're basically the same rifle.
I'm not knocking the AK or Kalashnikov's efforts, really. Obviously, the history of the rifle speaks for itself. But the man certainly wasn't in the same league as a firearms designer as John Browning. Not even close. Objectively, I'm not sure I'd even put him in the top 10 designers. His gun is a different story in terms of it's proliferation and longevity.