So would the Nerevarine, Champion of Cyrodiil, and heroes of Daggerfall, Skyrim, and Arena be Shezzarines? Or what about the Tribunal and Dagoth Ur since their powers came from the heart.
Shezzar isn't the only being who can be mantled, and not all heroes mantle someone. As far as I can tell, here's basically the way it goes down:
The Eternal Champion was his own man.
The Imperial Agent was his own man
Cyrus (from TESA Redguard) mantled to HoonDing
The Nerevarine mantled an ancient hero who had become powerful through the culture of the Dunmer. That hero's name was Nerevar. He was also an agent of Azura.
The CoC mantled Sheogorath, and had some times to Shezzar via Peleial.
The Dovakhiin basically just remembered his own inner power / divinity, and had the soul of a dragon.
The Tribunal are really weird. Their story involves time-travel paradoxes.
Pelenial, Tiber Septim, and Reman Cyrodiil were all historical Shezzarines.
Also, are the Aedra really planets in the literal sense or is it metaphor? Are the moons actually Lorkhan?
Those things in the sky are literally the Aedra, but they're only metaphorically planets. Nirn (the world of TES) is the only place in the Arubis (the universe of TES) where space really makes sense. Beyond that, everything is incomprehensible to mortals, and they just mentally "simplify" things into the night sky you see in-game.