mdd- You don’t understand that rishus is not always measured by how big the aveirah on its own, but by what it represents in the larger picture. He came to undermine the true emunah in a way that totally pushed out of the Jewish people. That’s why he’s in boiling excrement (Gemara Gittin) – he’s the waste product of Judaism.
The Halachic question is about a goy believing in “shituf”. That has no bearing on our question.