My guess is the out sound was delayed slightly, the band hear and play with their their monitor sound, that's aweird mix anyway, think single channel IEM feeds.
So DKM play live, their quick monitr mixdown is patched to Bruce, that's easy, perhaps a 100ms delay, noticable on video but he don't do video, he just plays with it. Bruce plays in sync on his end sound is sent back and mixed with the DKM sound, total delay before it's on ait, less than 300ms, Notice in the video that they have to pick if the video of Bruce or DKM is in sync with the sound, it's either or just like on a bootleg that shows both the stage and the big screen at the same time.
All network audio systems, like Dante or any other system, all use the same trick to sort sync over delayed connections, a world clock that is in sync between the sender/receiver on each end, that way the system knows how mch delay to add in each step to keep things in sync.
Summary: I don't believe DKM heard Bruce in their ear while playing but rather a clicktrack with the cues for when Bruce came in. The delay would be little enough to talk to each other but big enough to mess up playing.