Futanari is Japanese for Dual Form or Hermaphrodite. A Hermaphrodite has both Male and Female genitalia. Futa is short for Futanari. So for your characters to be Futa, they would need to have a Penis and Vagina. They don't always have Testicles, but they do have Dick and Pussy. Otherwise your Dickgirls are just Dudes with Boobs. Dickgirls are not Futas. They have their own terms. You can't tell me a Fork is a Spork.
Maybe their trying to use the Futa tag to bring in extra traffic to their game from other fan bases. But that's false advertising. Using Futa in this way is only for those that don't know the difference, or don't care. If you were allergic to peanuts and I gave you some calling them grapes, are you going to be upset?