When doing one of the Tribunal Temple quests the priest would tell you that Ald Sotha is the city of House Sotha, which was destroyed by Dagon, and the only survived member is Sotha Sil who is rescued by Vivec.

I am confused about this, seems doesn't make sense. I think in the perspective of the Temple the Tribunal are born gods, right? So if they are gods how can Sotha Sil be a member of a mortal house and being rescued by Vivec? I remember Sotha Sil and Almalexia fought with Dagon and defeated him in Mournhold.

I wonder if he is talking about the real history when the Tribunal are mortal. Though it seems impossible.

Even in real history the story sounds weird to me, like some sort of exaggerated stories that Vivec would tell.

According to the official Temple doctrine, the Tribunal were mortals who became gods through virtue:

"Temple doctrine claims their apotheosis was miraculously achieved through questing, virtue, knowledge, testing, and battling with Evil; Temple doctrine claims their divine powers and immortality are ultimately conferred as a communal judgement by the Dunmer ancestors [including, among others, the Good Daedra, the prophet Veloth, and Saint Nerevar]." (From "Progress of Truth".)

So it's perfectly normal for a priest to talk about the mortal lives of Almsivi.

The Jills are far from perfect. They cannot reconstruct time down to the beliefs and written accounts of mortals. And from the original timeline people's perception, is that they became gods, even when in this timeline they were gods from the very beginning.