It should be obvious to most people that it's trivially true that practice helps you improve skills that require manual dexterity.
I also think it's highly likely the anecdote is supposed to be illustrative rather than literal. IMO it's meant to be advice to students not to fret about how perfect their work is and that with time it will improve. It's just been distorted by the "one quick hack" culture of self improvement.
I’ve heard this story lots of different ways and assumed it was a fictional allegory. If it’s true, I can’t help imagining some poor student in the “quality” group at the end of the class, after the big reveal, looking down at their inferior ceramic bowl and going, “well, that was a waste of time,” and then maybe there's a sad moment later where they give up their dreams of becoming a ceramicist.
I also think it's highly likely the anecdote is supposed to be illustrative rather than literal. IMO it's meant to be advice to students not to fret about how perfect their work is and that with time it will improve. It's just been distorted by the "one quick hack" culture of self improvement.