This is a total and complete retcon. Seriously, Cassandra Clare really thinks her audience is as dumb as she is. We never saw Jace interrogating a demon at all! Yet, he still got the location of Valentine’s lair? I call BULLSHIT!! Clare knows damn well that she pulled this straight out of her ass.
Also, this is just pure laziness on Clare’s part. All she had to do was write a few paragraphs in the last chapter to make this work. Yet, instead of doing that Clare just thought that she could bullshit her way through Jace’s explanation. Nope! Also, I ask this a lot but it bears repeating: WHAT THE FUCK IS GOING ON WITH CLARE’S EDITORS? Seriously, this is another continuity error that her editor should have easily picked up on, but this just passed through on unchanged. Her editor should be fucking ashamed of themselves.