Did I? I count Talos from the moment the Numidium was activated using the Underking's heart as Mantella. Or, at the latest, after his conquest of Summurset. The prerequisites were completed at that point.
On the other hand, I use Tiber to distinguish the physical person from the god Talos. So we're fighting Tiber, but he's already Talos. That may be just me, though.
I feel like this whole Talos/Tiber business, the whole "He wasn't a god yet, but he was always a god" idea is intentionally beyond rational thought. It's like H.P. Lovecraft's notion that the names of gods weren't meant to be uttered by human mouths, so no human can ever pronounce it correctly. Like no mortal can correctly explain the point at which Tiber became a god, whether he was born a god, became a god, or ever died at all; see the age he was when he supposedly died. At least I can't figure Talos out.
As far as I can remember I was proposing a theory that Cyrus would join the Talos oversoul, and you said that it wouldn't work because he's fighting Tiber Septim, and not Talos. I believe your rationale was that he hadn't become Talos yet, so it wouldn't be possible to become a non-entity or at least a not-yet-entity.
Concerning Numidium, I thought that the Mantella was the one thing preventing Tiber Septim from becoming Talos, as in Vivec arranged the swap of Numidium knowing full well it would lead to Tiber unintentionally trapping his own soul through betrayal, which he did in fact do; leading Vivec to believe that he killed Tiber (seeing as how Tiber killed Zurin, a part of himself). With Zurin-become-Ysmir (who was already Tiber) trapped in the Mantella, Tiber's soul became half trapped. I imagine a piece of food tied to a string and swallowed, so that it's inside the stomach but also has a connection to the outside. When Tiber the man died a mortal death his soul was still tethered, and only after Numidium was destroyed (Zurin/Ysmir's act but also an unconscious act of Tiber to free his soul) was Talos completely free and able to become the full god.
But yes, by the time the Underking's soul was trapped in the Mantella all the prerequisites for all of them becoming the same were met. Still though that only explains to me
how they all became the same. It doesn't explain how they became the godly Talos. I made a thread awhile back asking when and how Tiber Septim became a god, but I never got a straight answer. More specifically I got too many straight answers that weren't the same. Under the incarnation rule, Tiber was born the avatar of a god, and he became the actual god Talos when he died, but some of the things he did in life would suggest he was already a living god. That's the disconnect I've never understood.