The author only says that a certain area of the brain tends to be larger in highly skilled musicians. Presumably this correlation also applies to highly skilled pianists, but he never specifically mentions the brain structure of pianists.

The author overlooks the possibility that the causal relationship could be reversed. Maybe having a larger brain area causally contributes to people becoming highly skilled musicians, not the other way around.

This is the cookie-cutter flaw of hasty generalization, where the argument draws a broad conclusion from too little evidence. The author doesn’t make this mistake. He draws a conclusion about musicians’ brain structures from evidence about musicians’ brain structures.

The author does address this possibility. He explicitly says that a certain area of the brain is smaller in non-musicians than in highly skilled musicians. The amount of music that people listen to is irrelevant.

The author never makes this assumption. He compares the brain structures of highly skilled musicians and non-musicians (people who “rarely, if ever,” play an instrument). He doesn’t compare highly skilled musicians to other musicians.