The misconception is that every betta is the same. Bettas are individuals with different personalities, like many cichlids. SOME bettas will accept tank mates without issue. SOME bettas will not accept any tank mates at all. Stating "male bettas are only aggressive to other male bettas" is false. Stating females are less aggressive than males, is false. It depends on the INDIVIDUAL betta. You don't know until you try whether your betta is okay in a community environment or not, so be extremely selective about what fish you try to keep with a betta and always be prepared to remove fish as necessary.
I bred bettas for years. I literally have kept and raised hundreds and hundreds of them. There is no blanket "they will" or "they won't" when it comes to bettas and tank mates.