Oh, I forgot to say this explicitly: an important reason why the trivial fix was desirable is that it was a clear-cut bugfix, making it easy to backport all the way to the 3.9 branch where BooleanOptionalAction was introduced.

Having this in place, your potential PR here might be a cleaner refactor without having to worry about backporting. Of course, if we manage to keep it small and isolated, we can bring it to 3.9 and 3.10 as well.