Jalen remembered being pulled into a pool of Darkness; being trapped in Alexandria away from his wife and son. It wasn't something he ever wanted to experience again, but he had his doubts about Medrau's sincerity, and dealing with the First Darkness before they absolutely needed to...

Though she had a point.

"I don't think a decision like this should be made just among the three of us."

Yes, he was purposely leaving out the Shadow.