The kefirah is believing in another, independent power besides Hashem.
The ramban is saying just that they can’t be blamed if they enslave the jews, because that was the job given to them. But if they decide to do more, they’ll be punished – but he doesn’t say that they can independently cause it to happen.
The suffering would have happened anyway, just someone else would have done it; Harbeh shluchim lemakom, etc..