Given that we've never seen any Trill symbiont override the gender identity of other joined Trills, I doubt this is the reason they're non-binary. Especially since we know they're human. I'm guessing that Tal ends up joined with Gray who has been mentioned as an unjoined Trill in promotional material. Then Gray and Adira become friends because Tal already knows them from their time being partially joined. Some of the trailers implied that they were friendly, so I imagine that's what initially causes them to become friends.