Assuming he worked his ass off is speculating as much as assuming it all never felt like work to him. I can attest that I've done what people thought was working my ass off but really felt like nothing because it was enjoyable and came naturally.
I remember reading a few times in interviews with people that knew him, that Hendrix basically never put the guitar down. I think in the sense the author is trying to convey, he "never worked hard" and "it just flowed" because he must have truly loved what he did, and he was doing something that he had a gift for. But from what I have read, he dedicated a lot of time to practicing.
The author is not trying to convey that he never worked hard. Believe it or not, you can both work hard and enjoy it. The author is trying to say he enjoyed his work so it never felt like work, therefore it just flowed.
I'm not speculating. Work isn't something you feel, it's something you do. It's documented historical fact that Jimi Hendrix played the guitar incessantly.
