I think this should be dropped. IMO it is a pedantic nit.  There is the everyday usage of the word "container" which has a reasonable plain meaning.  There is also an ABC that was unfortunately called Container (with a capital C) that means anything the defines __contains__.  IMO, we don't have to change all uses for the former just because the latter exists.

AFAICT, this "issue" for the argparse has never been a real problem for a real user ever in the entire history of the module.

So, unless there are strong objections, I would like to close this and we can shift our attention to matters of actual importance.