If you check his first story on Instagram at the moment there is a great shot where the lighting really showcases how far his arm comes away from the guitar when playing these types of single string phrases 
Anyone noticed themselves doing something similar in their own USX playing? I found my USX sweeping very fluid but always struggled with the purely alternate side of the technique… that being said I was always anchoring to the guitar 
Sadly it’s gone now but there is a similar view in his latest post:
Almost looks Batio-ish from this angle 
I was just giving it a go and it was feeling pretty good! Using my middle finger on the guitar body to help control the motion. I’ve experimented with a similar motion previously when trying to get a gypsy-style thing going but that was more supinated while this is a lot more parallel to the strings then I would of expected.
Maybe the motion is more trapped like this? Might make sense that he clicks into this style for single string lines?