The Glock is striker fired, not double action. He probably did not have a round in the chamber because Glocks do not have an external safety. For this reason, some people choose to carry them with a magazine inserted but with no round chambered because they fear an accidental discharge. Of course, if makes more sense to have a round chambered and ready but some people choose not to do so for the reason stated.
correction — ....Of course, it makes more sense to have a round chambered and ready....