Not sure I understand your question and probably wouldn’t be able to answer if I did! Here’s a try, tho…
He’s playing over the 12-bar blues progression (so, 1-4-5) and he’s improvising using the major scale of the chord that is playing at the moment. But, instead of single notes, he’s playing these 6ths. It sounds good because, well, Justin, but beyond that because he’s playing from the appropriate scale for each passing chord. It also sounds good because he chooses the right 2nd note to give the 6th.
BTW, it could be any chord progression, doesn’t have to be a 1-4-5.
Actually, watching the video again, I see Justin mentions the Mixolydian scale, so when I say “major” above, it probably should be Mixolydian. This means the 7th note of scale is flattened, or D# → D. In that case, the 6th for the D note would be frets 11 - 10 (strings 3 and 1).