Unless somebody has an employee list and financial statement from Amplitude, I think it is more accurate to say that the amount of interest and commentary they've produced from the gaming community (specifically, the 4X Historical gaming community) makes it appear to be an AAA game.
A lot of that, of course, is because there haven't been a lot of 4X historical games to comment on in the past. My impression, and that's all it is, is that they have been building up to this for years and have made some better decisions as to where to allocate their assets. Case in point: the animated Leaders of Civ VI, which have generated some discussion over in the
Civilization VI: Possible New Civilizations Thread and which I would argue are a Resource Sink for graphics and other resources that could be better used elsewhere, as
Humankind does.