How is it not a BTC fork?
Discussion
It has it's own codebase and whitepaper called CryptoNote (https://web.archive.org/web/20201028121818/https://cryptonote.org/whitepaper.pdf)
it's not a fork because it's a completely different codebase.
I'm not sure if this is true. I would bet there's some overlap in the codebase. If so, I'd say when you redesign a whole codebase and use some of it as it was originally, that still counts as a fork.