I'm not exactly sure. I think they mean ancestor in the spiritual sense, or cultural. More of a metaphor. However, it could also be that they don't draw as harsh a distinction between Aedra and Daedra. I mean, technically speaking both sets of "gods" are racially et'ada. I know what the Altmer say the difference between the Aedra and Daedra are, but I think it just depends on whether they are aligned with Padhomay or Anu. The dunmer are definitely aligned with the big P. I think that's what the "ancestor" thing is about. Merely a metaphor.
Actually, the dunmer believe they are from the aedra/daedra. And that Lorkhan creating the realm of Nirn and filling it with life, was treachery. That they were basically pulled apart from immortality and made into fleshlings. It's not a metaphorical worship. They worship the spirits because apparently in death, they ascend into what WE would call heaven. Or back into immortality. Death is basically an escape back into what ever peaceful slumber they were once in.
I think all life is of Sithis. See, with Anu, there was nothing. As soon as Anu made Sithis of himself, too piece together all randomness, Sithis became somthing. When you hear, "...before sithis there was nothing, " it means that Anu is basically so pure that he's to be not even there. If he/she were, they would've already been tainted. It's best to consider Anu a force so true it would be unwise for him to appear. So, Sithis, of Anu, is the father to the Et'Ada. To Anu, Sithis is a brother of sorts. When you say the Dunmer are of Padomay, it's more all things are of Padomay. I also think the Aedra are of Padomay too. It's like Akel kept the forces more primarily balanced, while Sithis focused more on the daedra. It's like, "Hey, Sithis is motivating corruption, I'll create the aedra."
But from all things there is an opposite counter part. Like Lorkan was an aedra. A trickster. The dark side of what was suppose to be a "pure thing". Of the daedra, is Mephala, Boethiah, and Azura. The "good side" in what was suppose to be an "unpure thing"
It's complicated. (understatement of the century)