Define your terms.

If anyone who has ever sung one or more folk songs is a folk singer, then Barbara Dickson is.

If a folk singer is someone whose primary output, professional or amateur, is currently folk music, then Barbara Dickson isn't.

By that definition Isla St. Clair was, then wasn't, and perhaps is again (I'm not sure what else she does, if anything). As long as she keeps getting booked for clubs and festivals, does it actually matter?