Expertise is applying the right amount of effort at the right time in the right place.
It is developed by the long slog of being good enough many many times, because feedback matters.
Good enough requires focus on the actual problem and avoids being proud of something that doesn't solve it.
Does your neice have competing opportunities where polishing actually blocks out?
My experience is that I don’t have 5 things I could do if I end with good enough. I rarely have 5 assignments due at the same time. So if I polish something, I’m actually just losing time that I would have spent on some marginal activity like tv, reading, etc.
It also is hard to evaluate opportunity and value, especially ahead of time. So focusing on a single experience rather than trying to increase quantity of experience may result in a lower level of impact than just polishing a single item.
In the case of my gf's neice, she is in an exploratory phase with a truly incredible amount of high-quality opportunities right in front of her. Losing attention/time to something low value would be "high cost," by my assessment.
When I write docs for my small team, I’m pretty lazy about it. If I write docs for a new joiner, I put in more work. If I write docs for external users, I’m both careful and I seek external review!
Writing for example, is often as much about the polish applied after initial drafting is done. Perhaps for an academic assignment in a class, that's not a big deal, but it would be a huge deal for... marketing copy, fiction writing, scientific papers, journalistic reporting, screen-writing... etc.
The other possibility, where asking for polish is an attempt to be helpful, not a power-play, is where the professor is attempting to train the student to produce better work in the field of speciality of the professor. This is actually part of the professor's job. In that case, it would be unreasonable to attribute malice, and instead it should be considered a simple case of two people optimizing for different things.
I’m going to ask an inappropriate personal question:
Did your niece recently choose to spend time on her schoolwork versus spending time with her family?