According to the documentation only the ArgumentParser has add_argument_group option, which is not true, the code allows me to add a subgroup to any group. The complete example is in issue 4608: https://bugs.python.org/issue46058

If add_argument_group shouldn't be used for a regular argument group,
I suggest the following change: _ActionsContainer.add_argument_group should return self if title is not specified and raise error if add_argument_group("...") is called on a group with title.

This is close to the originally intended, documented behaviour.

I'd still allow groups without title in mutually exclusive groups and vice versa.