Yes, it is http://www.imperial-library.info/content/etada-eight-aedra-eat-dreamer; and the Hidden Heaven is the Scaled Blanket, and the Scaled Blanket is Serpent.
You never asked about unstars. This was your question:
what makes Hidden Heaven the Serpent?
To which I replied with a source. No philosophy, no mystery:
With this nature I am invited into the Hidden Heaven!'
By which he meant the Scaled Blanket, made of not-stars, whose number is thirteen.
Well I thought the discussion had already set into stone that Un-Stars were, beyond a doubt, referencing the Serpent. I was merely asking for a somewhat simplified explanation of "Hidden Heaven" as that term is only used (as far as I know) in Sermon 33. Every time I read extended, broken up works in TES games (or outside the games on TIL), I miss terms and concepts. It took me a dozen or so readings of the Camoran Commentaries to understand them well enough, and I doubt I'll ever fully grasp Aldudagga. Excuse me for not immediately being able to extrapolate MK's lore puzzles.
Also, as much as I've read about Akatosh being insane, I've yet to fully understand what that means. Schizophrenia, though I don't think would apply to a god of all things. It's not like gods dopamine or anything. He definitely has a schismatic mind, but it's hard to imagine a time god, much less the time god, from which all temporal order originated being insane. I'd buy a "most of the time sane with exceptions" explanation, or even a theory that states that Akatosh created time, split in two, went nuts, and no longer has control of time but is merely a gibbering half-consciousness in the Aether, Shezzar retaining the sanity but not the body.
It may seem like an overly holistic observation, but how could there not be a link between Aka's insanity and Sheogorath's existance.
As Jygallag states, he is now free to roam the waters of Oblivion once again.
If my memory serves me correctly, stars are holes into Aetherius. A hole into Oblivion would represent an existential paradox, (although not so much after the Oblivion Crisis... possibly? I don't know.) An Un-Star, could be a hole into Oblivion, a hole into Sheogorath, the Sithis-Shaped-Hole, which was opened when Lorkhan's heart was plucked out. I know there's already extensive lore on the Serpent in every culture, but I'm just grasping for limbs here, trying to connect Aka's Divine Madness and Sheogorath's existence. An Un-Star could be defined in two ways as well:
1. The opposite of a star.
2. A star-like object/aperture that isn't, in fact a star.
Back to Jygallag, I find it odd that he would say he wanders the waters of Oblivion, just as the Serpent wanders the night sky. He would, in fact, not have his own realm but constantly live in the reflection of another self, ruling in his stead in a false realm.
Just some thoughts; stream of consciousness I suppose.