The video is designed to show direct opposition to what was previously believed. Previously, it was believed that dragons infected their prey, and then would follow weakened animals until they dropped from septicemia.
The same scientist in the clip here (Bryan Fry) is the one who discovered the combination of the hemotoxin and serrated teeth as a method for killing back in 2009. (He found the glands.) There is no question they express venom, the only thing missing was capturing images of them killing via envenomation in the wild.