Disclaimer: I haven't kept a betta fish yet, so not claiming any personal expertise! Just happened to watch a youtube video where they imported several hundred (absolutely stunning) bettas, and talked with the supplier, explaining their breeding set up and how they packed and shipped so many bettas.
The males were packed individually, while the females were in crates, with something like a hundred females together in each crate. The importer stressed how important it was not to mix and change the females into different boxes. He said each box had females that had been raised together since they were fry. That they were fine together as they were raised together, but that if you removed one of the females from crate A and put her into crate B, there would be a problem, and very likely aggression, from that female, and/or the females in crate B, who would almost certainly attack the outsider.
The bit that caught my attention (this was playing in the background while I did tank maintenance, so I admit I wasn't paying super close attention) was that he also said not to remove any fish from crate A, then replace them in crate A after more that a few minutes. 13 minutes, to be precise. (I have no idea how he arrived at the number, and I will try to find that video again to check it and link).
He handled a female to show her to the youtuber, and said you could remove one/some to view or examine them for a couple of minutes, put them back in, and it would be fine. But not to reintroduce them to the same group if they'd been separated for any longer than 13 minutes, because then the fish that had been away from the group became the outsider, and aggression happens.
My guess is that as you say, breeders don't usually have problems keeping the females together since that's how female betta are, especially before they've reached maturity. But since they're usually right at that cusp between sub-adult and adult- territorial- ready to breed mature adult fish, that their territorial instincts are beginning to kick in. So while in the wild, they're likely still in the same pool as their siblings, at the stage where they're gradually each leaving to find their own territory (males) or to find a mate (females), they're going to respond badly to outsiders. They are a solitary species, after all. Of course they're going to aggress when encountering another betta.
Betta sororities are a trend now. I personally don't want to try it. It isn't how they would live as mature adult females in the wild, and goes against their natural instincts. That's bound to cause unnecessary stress with the battle for a pecking order and the fish having no escape from the pressure of having to live in a way that isn't natural to them, with fights and deaths often happening. They say experienced hobbyists only because they need such careful monitoring, and because the pecking order is always going to be subject to changing on a dime; with potentially disastrous results.