There’s been some Friday night kernel drama on the Linux kernel mailing list… Linus Torvalds has expressed regrets for merging the Bcachefs file-system and an ensuing back-and-forth between the file-system maintainer.
The question was “How do you define GPL compatible?”. The answer to that question has nothing to do with code being split between files. Two licenses are incompatible if they can’t both apply at the same time to the same thing.