You could argue that the trait we call intelligence is about how quickly efforts to learn something new result in new capabilities, rather than whether effort to learn something results in new capabilities at all. And from my (admittedly shallow) familiarity with the field, that argument seems to be substantiated.

Problem is, even if that's true, it's not a particularly useful truth to most individuals. A growth mindset is useful -- unless you're particularly unintelligent, it's probably true that your efforts to learn will have results that make you more capable, and effort can compound on effort. You should probably act as if you can become more capable than you are, even if there are degrees of inborn intelligence.