I'm not certain it was a mistake. I think he is using it to control sustain. Going from memory here, but I think he uses the same Gate channel for the Clean Boost scene and the Solo scene. The sound chokes out on Clean Boost scene, but the Solo scene works well (at least for me).
If it's giving anyone problems, just swap channels, tweak the threshold, or bypass altogether. I noticed there was about a 60 db difference in the threshold levels between the channels, so that's probably the cause. Most annoying for me was that the Clean Boost scene wouldn't pickup any notes played on my high E string unless I really dug in and then they sounded chopped off because of no sustain.