Midgardsormr and Omega do, indeed, seem as though they're on a power level similar to that of the Ancients, so it's not far-fetch to speculate that they are the equivalent of unsundered beings.

And when discussing "sentience" it's worth mentioning that most animals in general are sentient. It's not a measure of intellect - it's a matter of being able to feel sensations. It was once believed that only beings with souls (i.e., humans) were sentient - that all other animals were purely instinctual, basically machines made out of meat. This made mistreatment of animals justifiable - that screaming animal wasn't in pain, it's simply reacting in the manner that it's programmed to. Today, sentient creatures are basically any creatures that can feel, but people STILL misuse the term to refer to humans only (or, in fantasy and sci-fi settings, intelligent aliens, as well).

A better word to use is "sapient", in my opinion, which indicates the ability to deeply understand things. It's what separates us from animals (well, most animals, anyway). Does it separate the races of Hydaelyn from the Ancients? The Ancients may well understand things with GREATER depth than the mortal races do, but that doesn't change the fact that the mortals are sapient, as well. Emet-Selch doesn't really have a leg to stand on - and I think he knows it, himself, but is in denial. It's why he's tried so hard to connect with mortals from time to time over the millennia - and the latest disappointment was the last straw.