they very much THINK is toxic
what
I don't see the confusion.
They think she was knowingly three-timing them behind each others' backs, which most people will agree is a very bad thing to do and quite toxic. She wasn't, but they think she was, that is their POV of the situation. The entire premise of the plot so far is that she fails to clarify this so disastrously over-the-top that it's supposed to be funny.
they dont really, they arent crazy enough to think what a little girl said 7 years ago is binding or that someone of that age can even be in a serious relationship