From what I can tell, Radixashen is in Athel Loren. Can't figure out anything else about him but that's where he's at. So Deathfang is saying that the Goddess he made a pact with when he joined Athel Loren was Rhya. Now, you can argue that the elves of Athel Loren for some reason don't worship that Goddess, or that they worship her on top of Isha, but I think that's the natural conclusion. It is all very strange, and I do think we should seriously consider the possibility that he's just straight up wrong.
Actually, Deathfang said "Draugnir joined with your Gods, but Radixashen joined with Rhya", indicating both that Rhya isn't Isha and that when Radixashen joined Rhya he didn't join with Isha. I think that indicates that if Isha is present in Athel Loren at the very least she wasn't there when Radixashen...joined with the forest? I have no idea.
Well, the thing is that the arrival of High Elves and pact with the Forest is probably predated by the Dragons joining with High Elves.
I can't claim it as fact because there is no hard data, but i am pretty sure that Draugnir bound his faction of Dragons to Elves sometime in or
before the reign of Aenarion. If we assume Radixashen pledged his alegiances around that time, then he predates the arrival of Isha through Ariel and Orion by several centuries, if not outright millenia, meaning that Rhya was already there and then the elves, because they obviously won't give up their gods just because of collonization or later little things like schisms, brought Isha into the mix.
Athel Loren itself is also probably its own entity, so its entirely possible that the ties there are something like
Isha + Kurnous(Ariel+Orion) -> Athel Loren(Spirits) <- Rhya(Radixashen)
Im just gonna assume there is an overlap because both have dealings with that particular forest, who essentially acts as a third entity, but they are not directly related.
Its like Durthu, isn't it? Durthu owes no fealty to the Elves and Isha except through the ancient pact they've made, but he certainly doesn't worship Isha and he is far more a representative of the actual Forest than she is.
To put it in entirely different way, Isha is goddess through the mortal tribe of people that live there, Rhya is the goddess through ... something, been a while since i read her domains, and they are both tied to the forest itself through their worshippers/ representants. But it doesn't need to necessarily mean that they are the same deity.