That's just a thread where a bunch of people speculate. No one provided any sauce of any kind.
Hit said that if Aizen had ten Vasto Lorde Arrancar, then Soul Society would stand no chance. At no point has anyone stated anywhere that Aizen has even one Vasto Lorde on his side, and the one time Aizen himself mentions it, his wording makes it sound as though he's still trying to
find the Vasto Lorde, and that it's an ongoing project. He doesn't seem to have any on his side yet.
New question: What is he?
A Vaizard. And if you're referring to what he turned into to kick the shit out of Ulq: a superhollow.
I don't think any of the other Vizards can "transform" like Ichigo did.
Yeah they can. Aizen explained all this way back when, when he first saw Ichigo use the mask in Soul Society. Under the proper circumstances, a Shinigami can turn into a Hollow, and a Hollow can turn into a Shinigami. When this happens, the individual who turned becomes exponentially more powerful by entire orders of magnitude, and 'attains a power that is beyond what [a natural version of what they turned into] can wield.'
That is to say, a Hollow that turns completely into a Shinigami will become a supershinigami, and be far more powerful than any normal Shinigami. The reverse is true for Shinigami, as any Shinigami that turns completely into a Hollow will become a superhollow, and be far more powerful than any normal Hollow.
The Vaizard and the Arrancar are all incomplete, because none of them have (to our knowledge) fallen totally to the other side. Stark may have fallen (or at least come really, really close) given his powers, and it's pratically a given that Ichigo fell during his fight with Ulq.
Now, what we
don't know is if becoming a supershinigami or a superhollow is a permanant transformation, or if it's just a massive, massive powerup to the individual's opposing side's abilities.
If it's a permanant transformation, then we likely haven't seen a genuine version of a supershinigami
or a superhollow. But if it's just Ressureccion/Mask<sup>10</sup>, then both Stark and Ichigo are potential candidates for supershinigami and superhollow, respectively.
It'll be interesting to see what happens the next time Ichigo tries to put on his mask. If I'm right, then the mask may turn him into a superhollow now, just like how Stark's Ressureccion
might be turning him into a supershinigami.