Racism isn’t about perceived ethnicity specifically, it’s about prejudice toward an ethnic group regardless of whether you can perceive the ethnicity accurately.
When someone racist toward Arabs discovers that someone they didn’t think was Arab is Arab, then demonstrates prejudice, obviously the perceived ethnicity doesn’t matter as much as the prejudice of the idea of being an error.
I don’t see the disconnect between anti-Semitism and perceived ethnicity, or how such a theoretical disconnect removes racism from anti-Semitism.