Macfuse itself has been closed source for a long time. Whether their EULA is compatible with the LGPL though, I don’t know. (Also, the old question: are APIs copyrightable?)
And API copyright aside, how would one prove it’s only API compatible and not using the implementation as well without also open sourcing their implementation?
It would almost be better to be similar but not the same. Then everyone else can just do subtle ifdef changes to their fuse code.