There is more than one kind of relief
The rules provide several distinct routes. One covers understated tax you did not know about and had no reason to know about. Another allocates the balance between the two of you as though you had filed separately, which is aimed at people who are divorced, separated, or no longer living together. A third is a broader equitable route for situations the first two do not reach, including balances that were correctly reported but never paid.
Which route fits depends on the facts, and the request itself covers all of them, so the analysis is about which argument leads.

