I personally don't care for Hearteaters, but that's why the concept of apocryphal splats is nice. I don't have to use them or think about them. If someone tries to use them as evidence to argue some point about the themes of the setting or the nature of the Exalted I can just say "Hearteatwhomst? Those guys don't exist in my reality."
See Gaz, you're overestimating the quality of the line's historical writing again. It's not just a fandom-side thing; I can pop up Caste Book: Dawn and there's Lyta, brainwashing hundreds of mortals into blood-crazed fanatics to throw against Dragon-Blood as sacrifical hordes to drown them in a sea of bodies!
The term 'Primordial' actually has been discontinued. It doesn't occur in any of the extant Ex3 books, except as an adjective (that is to say to mean 'old,' not anything related to the Yozis). But 'enemies of the gods' is indeed kind of burdensome and vague. As a result, the Sidereal manuscript has introduced the term 'Ancient', in lines such as 'Whether forged by the Ancients or the gods,' or "Outandish rumors claim that the dweomerforge is itself a cast-off fragmant of one of the Ancients.'
As for why, I suspect that it's to dissociate whatever Ex3 is doing with the Yozis and the Divine Revolution from the Primordials. The term 'Primordial' has a lot of baggage; we know who the Primordials were, we know how they functioned, we know them down to the specifics of their biology, we have an entire origin story for them... So just retire the term and call them something else that is slightly different enough that it could refer to a broader, vaguer category of beings that don't have all the same commonalities.
This does kinda run into one of the issues the 'no Charms to rule justly' thing has always had, which is that Charms explicitly make people superhuman at figuring a lot of stuff out, and that's apparent in the James C. Scott comparison. There is no Charm to decide wisely, but there are absolutely Charms that let you identify at a glance that the allegedly 'primitive' agriculture of your people is actually optimally designed for the nature of the soil you have and if you replace it all with massive wheat fields you will destroy the soil and everyone will starve. There is no Charm to tell you what a good ruler is, but there are absolutely Charms that will tell you that taxing households based on their number of windows is fucking stupid.
Solar Charms are very good at obtaining information and predicting outcomes. They can't make you a good person, but they strip away a lot of the mechanisms that make states awful by necessity of ensuring their functioning. They tell you how to efficiently obtain taxes, how to spend taxes effectively, what tax burden your population is capable of comfortably bearing, how to placate your warrior caste... Yeah, if you suddenly decide that trees are personally offensive to you and to go on a crusade to raze every forest, bad things will happen, so... don't do that?