I'm trained in both mental health nursing and counselling, there are overlaps, I trained as an RMN 23 years ago and we had two years of our three years training focused on PCT, CBT, Humanistic, group therapy and I went on to be trained in DBT.
I would say it's incorrect to say mental health nurses don't do any therapeutic work, it's vastly different across different roles and certainly there is therapeutic work to be done in all of them. It is very different to the counselling work that I do, because the setting and intention is very different. Someone trained only as a counsellor would find it very difficult to work therapeutically with someone in acute psychosis actively seeking to harm themselves for example, whereas an experienced mental health nurse could.