Considering the term "Heritage Poultry" is currently under hot debate, I'd say they are if you want them to be.
That's about how much weight the terminology carries these days!
There are a lot of breeds very deserving of the term, IMO, that do not fit the one definition that has been laid forth by the American Livestock Breeds Conservancy, which, though admirable in many ways, is not an authority on poultry terminology or "branding". No more relavant organization has yet granted their approval of a definition, so, IMO, it's fair game.
Do it justice. If you feel Welsummers have a legitimate right to be dubbed heritage, than so be it. Do your research, and defend their title as such.