User blog comment:Thetwindler/God vs. Jack/@comment-3045936-20190423031951/@comment-3423279-20190423212830

Well that was just a claim the shadow made about the Empty, and it is false since Castiel was straight up revived like 3 times by God, Nick was able to awaken Lucifer pretty much just by asking, and Lucifer obviously knew how to escape the Empty as well. More to the point apparently Jack can't just make angels like God can so there's that limitation on Jack too.

If anything, it seems far more likely the Shadow entity was lying to get Castiel back to sleep and Jack is more powerful than an archangel, but less powerful than God/Darkness/Death-level entities even at full power.