I mostly agree with you, I don't do SR myself, but I do think the benefit of spaced repetition is solid for medical students/researchers and language learners.

Also some people just find it fun to go through their Anki deck instead of doomscrolling while on the subway or waiting in line. Whether there's any real benefit for that person is debatable. It's “fun“ in the same way going to the gym, or drinking kale smoothie is fun.

The key for improvement is deliberate practice, and one component of that is "working really hard" which you've pointed. But this is still too vague to be helpful. For those interested in the science of expertise, I highly recommend Peak by Andres Ericsson.