Well, that would sound logical, although I think that God Slayer is something Hades came up with on the spot. I mean, if God Slayers really existed they would have enough power to take out Zeref (well, he's close do godness anyways) and since gods as such in FT don't exist (so far!) I could assume that Dragon Slayers are still the strongest slayers out there.
Natsu is rather young, we've seen him synchronize with dragon magic by injecting scales into his physical body and therefore becoming truly powerful (he did knock out Zeref's incarnation after all) so no wonder there might be some slayer being several levels above him...
Besides, like I said - dragons do exist in FT world (or existed?), gods... not so much. I think that if they said DEMON slayer I would give it more credit.
Just a thought, totally out of the blue... Do you think there's something between Erza and Natsu? Or might possibly be? I can't get that scene out of my head where Natsu nearly got killed just to rescue Erza from La'cryma. I sincerely doubt he would do as much for Gray...