I'm not much for recording video, but I type 60 WPM on a good day. More like 50 on an average day. But I wonder what he means by "burst" (is the link to the right part of the video?). For a burst of a few words I may be 15 WPM or more higher than the mean WPM for a paragraph. If he's talking about that kind of burst, it may be his average WPM is not so incredible (though still admirable).
Of course, some people are just fast. I don't think I've ever in my life managed a sustained 120 WPM on a standard keyboard; 110 WPM is about my best. Yet I know there are much faster typists than me in the world. There are relatively few Twiddler typists in the world, so it's believable that 120 WPM twiddlers wouldn't be much less common than 120 WPM QWERTY typists if the sample were big enough.
It also bears mentioning that (using myself for reference) I've been typing on a standard keyboard for 20+ years, and the Twiddler for 2 years, and of course not very consistently before I built enough speed to put the standard keyboard in the closet. Somebody else may know how long Starner has been using the Twiddler (10 years at least it seems), but it seems he's been doing wearable computing for as long as I've been typing. I'll have that much experience chording in 2035. I don't know that I expect to reach 120 WPM by then, but I think I can hope to be near my QWERTY speed.