Correct because as documented it calls invalidate_caches() on meta-path finders and the one that handles sys.path calls invalidate_caches() on the finders found in sys.path_importer_cache() (as documented by https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.invalidate_caches). Since None isn't a finder it doesn't do anything there as there is no method to call. So the "full of lies" comment by the OP on the mypy repo is over-the-top/incorrect.

Now tweaking https://github.com/python/cpython/blob/521995205a2cb6b504fe0e39af22a81f785350a3/Lib/importlib/_bootstrap_external.py#L1180-L1186 to delete entries in sys.path_importer_cache() that are set to None is straight-forward, but I don't think we can backport since it would be going against what the documentation says it does.