That could mean anything.
For example, while we're speculating.
That could be some sort of understanding between them that Bella would vet anyone Jill has her eye on. Perhaps out of the need to care of someone, maybe like treating Jill as her little sister.
Or simply that Bella could have been hired by Amelia to 'look-out' for her.
Or Bella is referring to how they almost argued over a 'guy' (Hoes before bros in their case). They could have a pact of friendship above all else.
Or a promise to never keep secrets from each other. And neither of them was actually keeping secrets from the other, but that situation was a simple miscommunication.
Or as you suggested, the possibility could exist about them having been intimate with one another.
I'm not discounting your suggestion, but the point is, that conversation could mean absolutely anything.