When checking links BLC seems to check the file exists but doesn't check the fragment id:
Getting links from: http://localhost:59319/
├───OK─── http://localhost:59319/#an-invalid-id (CACHED)
├───OK─── http://localhost:59319/foo.html#an-invalid-id
├───OK─── http://localhost:59319/#a-valid-id (CACHED)
└───OK─── http://localhost:59319/foo.html#a-valid-id (CACHED)
Finished! 4 links found. 0 broken.