That areticle is kind of right. If you only have two females, then a sorority isn't going to work. But if you have 5 or more, then the females can live together. If you take into account the occasional female who is too agressive for a sorority, then female bettas can't be kept together with 100% success. It would take skill to set up a sorority properly. It's not like you can toss 7 female bettas into a 20 gallon tank that doesn't have any hiding places and expect success.
Too bad the article doesn't explain it that way. They make it sound like females together in any number would be bad. Females can be kept together if you do it right. But it does get a cookie for mentioning heaters and space. :)