[monsieurbanana]

In the aforementioned anecdote, the pianist was struggling with a new composition, not general piano skills. Which would make sleep a catalyst for making new neural pathways for that specific composition, not how to be a pianist in general.

That's my take as a layman.

[ehnto]

Learning a new piece usually requires learning novel movements, stuff you've never done before. Maybe you've played a C major before, but you don't always use the same fingers depending on where you're coming from and where you're going, maybe the cadence or intensity is different, and so on.

[slver]

Still, I can witness myself if I haven't slept well not only my ability to figure out complex programming code suffers, but the mere ability to type fast without a ton of typos.

Otherwise I agree after sleep, what we learn seems more readily available.