That's true. I figured she just didn't have a way to tell them, since Hermes was also traveling with them to Amaurot. It would be more suspicious if she approached Emet and Hythlodaeus and then they wanted to know the truth, including why Hermes wasn't there. Though I'm more inclined to believe what Raven2014 said about she wanted more time to combat a known disaster than an unknown one based on what we told them about how the Final Days was inevitable. It was also unknown how much actual time they had before Meteion caused the Final Days. However, we defeated Emet (which showed we had power to contest with the strongest of the Ancients), so she probably thought it would be better to have more time to plan things out and rely on the ones in the future. She also tested us too. If she didn't have the belief, she probably wouldn't have saved her power for the Final Trial. The other thing to note is that she believed in us probably because we can use Dynamis and the result of the Final Days caused the Ancients to lose control of their Creation Magicks (aka control over aether), so the ancients weren't exactly a reliable source to contend Meteion with.
The cutscene in Thou Must Live, Die, and Know.
You can tell everyone ignored Venat's words about moving forward. They simply consigned themselves to offering themselves to Zodiark, depending solely on Zodiark to deliver themselves out of this fate. She disagreed with the Convocation about life, but I'm pretty sure this cutscene was referring to all life on the Star, including the Ancients. Her facial expression went from shock to grit and grief, before she draws her sword. "Let there be no way back. From this temptation, I sunder us."
This temptation, refers to relying on Zodiark.