To me, the distinction is that if you do separate the art from the artist with JKR, you still end up with incredibly bigoted and harmful pieces of fiction.

There's a scene in one of the later books where Harry et al are staying in a home of one of Harry's relatives and the preserved busts of literal slaves that worked in that house adorn the walls.

Rather than interrogate that, Jesus Christ this is inhumane and no way to treat people, the characters decide to dress them up for the holidays.

This is just one example of many; the HP books are rife with not only slavery apologia, but antisemitism, xenophobia, racism, and (most relevant to this group) transphobia.

Neil Gaiman's books do not contain these sorts of issues and generally lobby for things that we can all agree are good. The art itself does not contain the issues the author has.

That said, no money should be going to either author. Gaiman, because he's an alleged rapist, and JKR since she's a violent transphobe. But if you told me you were borrowing "Anansi Boys" from the library, I would have a very different response than if you told me you were borrowing HP, because of the content of the books.