Imagine getting outflacked from the left by a normal green socdem when you call yourself a democratic "socialist"
The reason why voting Green is productive(ish) is that if they get a high enough % than they get certain rights as a "legitimate" party. Certain federal funds and ballot access are the basic ones. I think if they get a certain amount then they'll also be included in the presidential debates. If they ever get that far it may just tip their legitimacy enough to make them a viable third-party.