I'm going to make myself a lot of enemies here and claim that Civilization has never been historically accurate. It's never even really tried to IMO. It's history flavoured. And not even the natural flavour that comes from squeezing a fruit, but the artificial kind made in a factory which is only a mimicry of the real thing. And that's perfectly fine, great even.
I think this because the fundamental systems of Civilization (any entry in the series) are completely alien to history in innumerable ways: a completely invented geographical setting, all civilizations starting at the same time in the same state while 99% of the map is utterly devoid of humans, a centralised program of scientific research in the stone age, troops taking decades to march in and out of a city, archers shooting over an entire city, the same thing speeding up the construction of both tanks and libraries, having complete control over every job of every individual in the civilization, culture bombing, immortal leaders, making plans that last millennia, etc... In fact, I doubt there's a single game mechanic in any Civilization game which is historically accurate in its details, or a single historical process that's accurately depicted in Civilization on anything more than an abstract, allegorical level. While a lot of my love from history grew out of playing civilization as a kid, it also "taught" me lots of things that I've had to unlearn later as I got deeper into history, eg: that sword beats spear, or that technological progress has always been the main determinant of a nation's success.
And yet past Civilizations have been fantastic, immersive games! Historical accuracy is thus clearly not required. That's not to down play the importance of historical "flavour" and immersion. The latter is vital in any game, and the former is a cornerstone of Civilization and does contribute to (without being the sole factor for) immersion. But the historical flavour doesn't do that by being accurate. My theory is that it does so by making call-backs from the fictional world we create in-game to real-world history that we already know about, and this call-back immerses us in the mood and feelings that comes from thinking about historical things. So when you build the Pyramids, you think about ancient Egypt and that immerses you in the feelings that you associate from ancient Egypt - it's unaffected by whether the pyramids are a magic font of granaries, or if it lets you enter a representative democracy, or if they make builders magically be able to build more farms before self imploding. In that sense, the historical flavour plays a similar role as the tech quotes. [Which, incidentally, would explain why that awful Civ6 quote about Roman air conditioning got so much hate - it's like biting into a raisin when you were expecting a chocolate chip.]
Which I think circles back to the perennial topic of why a lot of folk don't like the idea of civ swapping, and I'll be bold enough to venture that some didn't know themselves (or couldn't express) why they don't like it. It's not that it's ahistorical (for all the reasons given above), but that it breaks the historical flavour. They wanted to play an Egypt-flavoured game, and 1/3 of the way through they have to switch to a Mongolia or Songhai flavour: they wanted chocolate ice cream but were forced into Neapolitan. To me personally, the idea of playing with a Civ-flavour that changes to match the time-flavour of every era sounds pretty cool. Yet I do still empathise the loss of having a single Civ-flavour the entire game. [Not that history-flavour is not the only thematic or attachement-related part of the game that is affected by civ swapping.]
To be even more conjectural, I wonder if this is the reason Humankind was disliked (I never played it myself), outside of anything to do with the game's mechanics per se. It's that it lacked this historical flavour. People did call it bland after all. More precisely, its historical flavours were never strong enough and long lasting enough to trigger this immersive call-back from the game to real-world history.
I'll stop pontificating here, and apologise for stretching a tenuous food-based metaphor for quite so long...