Female bettas can sometimes coexist, but it is tricky. They are still aggressive, just less so than males. Their aggression can be handled in larger groups - meaning their is a hierarchy to it. There will be one who comes out on top as the dominant female, and in order to ease the aggression on the other fish it is generally recommended that female sororities have at least 5 together. I only have 2, and that means that there would be one dominant (aka bully) fish, and one submissive (bullied) fish. The stress on the bullied fish would be terrible. I also only have a 5 gallon tank, and in order to keep 5 bettas together I would need minimum of a 10 gallon.
It also depends on fish personalities. Sometimes the females can be as aggressive as males, and if you happen to get more than 1 very aggressive female in a sorority they might not work.
Basically, you are right that females can coexist in one tank, but it is definitely a risk no matter what - they are still bettas, and less aggressive doesn't mean not aggressive. Maybe someday I would try a sorority, but I'm content to watch them interact through the divider and know that they are safe.