Agreed regarding the false dichotomy. I don’t share my former teacher’s views. For any language, especially one with as vast a speaking base as Mandarin, I think there can be multiple “standard” accents. I’d imagine that most native Mandarin speakers might find a general Northern accent (not necessarily a strong Beijing accent) to be the “most standard” and that a general Southern accent (Shanghai metro, Wuhan metro) or a Taipei accent (as opposed to Southern Taiwan) to be generally acceptable “standard accents” as well.
Personally, I would also think that accents that many consider “non-standard” are equally acceptable in my own estimation. I know that many listeners find Canto- and Southern Taiwan accents to be somewhat deviant, but I think there is a charm and beauty to them as well, and I can’t help but feel there is some level of chauvinism involved to call them non-standard.
Of course there is a subjectivity involved in any of this. If there is no such thing as “standard” then how might one consider ABC or other “foreign” accents in Mandarin? Are they just simply another kind of standard accent, or are they “objectively” just “wrong”? I have no hard feelings on this, just musing.