Zodiark's Codex entry what do we think about it?

[Cleretic]

That said, it's frustrating how the text talks about Amaurot because it feels like the writers can't decide if they wanted it to be the only real culture of the Ancients (how EW and the grapes short story seems to lean) or just one of many (as ShB seems to lean). The Codex entries in particular seem to treat "Ancients" as synonymous with "Amaurotines" and the Convocation as some sort of global government in defiance of Debate and Discourse's worldbuilding, which is something I find extremely annoying - my headcanon is that Amaurot became the global government by virtue of being the last one standing, but that's a stab in the dark. So it's difficult to say what their relationship was to other nations, and whether they really were "provinces" or if Amaurot was just the Sharlayan of the time and a little conceited about itself.

I never actually read it as implying other nations. I think the main thing you're thinking of in Shadowbringers, by your mention of it, is this one?

Loquacious Amaurotine: As to the matter of what subject we shall debate today, I propose the recent calamity which has befallen our friends across the pond. What say you? The singular point of contention is, of course, whether or not Amaurot should intervene on their behalf. I believe we should. The scale of the disaster which threatens that distant metropolis is of a scale heretofore unseen, and so equally considerable resources must be committed to counteracting its effects.

Amaurotine Firebrand: I disagree. The scale concerns me less than the nature of the proposition itself. Who are we to unilaterally intervene in the affairs of those half a world away? Are we to be the saviors of one and all? Such arrogance may well lead to our own downfall.

The main thing here that suggests 'different nation' is just raw distance; it feels fairly unlikely that a city 'half a world away' could be in the same nation. However, that's not only completely valid in certain contexts, it's also been true in the game's setting; the Garlean and Allagan Empires both had pretty far-flung colonies, and while I can't imagine either having that conversation because of their worldviews, I could see them talking about those further-out colonies in a similar way. (Also, if Emet did indeed design Allag to be reminiscent of his homeland, as suggested in a couple areas, we can therefore at least assume Amaurot to be a very large nation.)

All that said, I think in the overall view of the narrative from a Doylist perspective, we can definitely recognize Amaurot as the only relevant Ancient culture/nation. It's the only one we hear about and the only one we have any clear, non-circumstantial evidence of; whether they were a one-world government or not doesn't really change any part of the story as presented to us.