Admittedly, while I lean toward that being correct, I'm not entirely certain. We don't know exactly when Serini showed up to the cave, so she might have overheard that and, given her feelings about the Order's tendency to blow up gates, decided that stopping them now was definitely the course of action. If they'd just been scouting outside the tomb and talking about waiting for the right opportunity to catch Xykon, maybe not. But I think their track record probably gives her the cause she needs to move against them either way, yeah.