Well Venat was an ancient. She knew them. She was also a former member of the convocation. She knew the convocation members. She was also the former Azem. You know, the ancient who probably saw the most of their world.
She considered they had no chance. This whole "She did not tell them, if she did they would have bla-bla" is just whishful thinking from ancient fans. They would have probably failed. If they would have even believed her.
And there's also Hermes, their best dynamis expert. What would he have done? Too many variables and just a stupid risky decision.