That would mean that e.g. new_list.extend(1, 2) would also work. That could be useful but it’s not what this PEP is about.
In general, this PEP thread seems to attract a lot of ideas for related things that could also be changed in the language. Let’s keep this discussion focused only on the concrete proposal in PEP 798. If you have some other idea, open a thread in the “Ideas” category.