I would argue the conclusivity of this statement. Supergirl being name-checked on Black Lightning doesn't necessarily mean that she exists as a real person.
And the same thing applies to Vixen.
Of course we can't say anything for certain yet, but it's implied. When Lynn joked to Jennifer that she was Vixen, Jennifer seemed convinced for a moment, which is unlikely if Vixen is fictional in that world.
Besides, it doesn't matter. The concept of Supergirl exists there, which is incompatible with Earth-1 regardless of whether she's real or fictional. If there were a fictional Supergirl on Earth-1, then Barry Allen would've recognized her as a fictional character brought to life when he first met her. Instead, he'd never heard of anyone Super.