That's not what Dorian says. He specifically says they went there in search of the Old Gods. And Cory calls it the "Seat of the Gods." The lore has changed.
Then I'm fairly sure Dorian is wrong, or at best giving a Tevinter version.
Someone correct me if I'm wrong, but I believe Coryphyfish references Dumat's direction in DA2. Considering how very well established it is that the Old Gods were buried underground (and physically present on this side of the veil), there's not much sense in looking for Dumat in the Golden City. Not unless the old god bodies and the old god souls were somehow trapped on different sides of the Veil.
(Which is possible as a theory, I suppose, but doesn't seem to have much/any support in any of the religious lore or supporting ancient mythos that I can think of.