If we go by in-game mechanics, he seemingly can endlessly revive like the other Demon God Pillars can do so it primarily depends on if you go with the fact that Nero either caught Goetia offguard, Nero overclocked her Imperial Privilege or she was using her potential powers of a Beast there.
He can, but conversely we see him standing there unaffected by the torrent while everything both in front and behind him ends up torn up, melted, and radiating fire when the attack is finished.
Does off guard really work here? Not exactly like Goetia was expecting either Clarent Blood Arthur or Dead End - Airgetlám when they struck. Imperial Privilege would probably be the go to, but that feels like its being stretched thin given what he had to tank through. Recall, each band of Ars Almadel Salomonis is equal to Excalibur (though I suppose we don't know how sealed we're talking here, could be with no seals undone I guess) and there is 100s of millions to billions of them. Beast shit would have been detected by Chaldea, right? It certainly detected her oddity in Septem IIRC.
FGO has been really on the nose that Nero is really off ever since Septem(Being able to interact with Servants due to the excess amount of Magical Energy she has unlike Drake who needed a Holy Grail to even get that far).
She is, but its still kind of glaring that she's hitting harder than higher end NP with her normal strikes here. It's not even like she's in here NP's space that seems to boost her to higher levels.
And it would explain her taking Excalibur Galatine twice and not dying by Gawain in Last Encore.
I mean, I figured that was explainable by Imperial Privilege myself
The game implies that the gap between a Grand and a Normal Servant is big... but stuff like Atlantis where Caenis couldn't be injured by Super Orion does make it suspect in how big that gap is... but at the same time, it seems like Orion wasn't really using any of his Grand Servant Saint Graph or Magical Energy in the slightest either...(Then there's King Hassan and Romulus Quirinus which actually goes against Super Orion's showings in Atlantis, even Noah ends up doing sheer bullshit despite being weakened to shit in Arcade)
So you could go with them over-performing considering how massive the stakes are at that point.
I mean, the gap can't be that big. A Grand's E in durability still gets impaled on Goetia's fist per Solomon while Mordred, Bedivere, and Nero remained intact. Nero's likely explained by Imperial Privilege, but Mordred and Bedivere represent an A and B respectively. Even taking their armor into consideration as if it was on the level of Gilgamesh's, you're still not talking a gulf that wide in relative terms (they want to convey some kind of disparity akin to the Excalibur vs Ars Almadel Salomonis comparison, yet you're left with something that's not likely to be much more than an order of magnitude or 2 higher between the upper end of the normal servants and the lower end of the Grand Servants)
Kotaru does have an NP that would boost his STR and other stats but I don't think he used it in that scene though. Again, there may not be much of a massive gap between Servants. I forgot what suit Ritsuka wore and how specialized it was though.
I don't ascribe to having much of a gap in general, its just more we know Quetz can at her best force a Beast to bend a knee so to speak. Obviously wasn't using that here, but all the same.