“Separate the art from the artist” is an idea from literary criticism: it’s about giving you permission to talk about what the art _does_ without having to argue about whether the artist _intended_ that effect.

It’s not a moral framework! It doesn’t say anything about how to feel about buying a product, art or otherwise, that supports creators with odious opinions.

It’s not incongruent to be totally subscribed to the Death of the Author and also not want to buy any Harry Potter stuff.