It may be argued that we now have mouses with 5+ buttons, when Engelbart himself scaled back from 5 to 3 for the mouse hand, and we're now used to keeping one hand on the keyboard where we commonly do two- and three-finger chords, but there's a certain level of facility that I think can still only be realized with a dedicated chording keyset.
Engelbart's keysets could also provide two-way communication (by puffing air under the keys) to do things like prompt for certain responses when mousing over certain screen elements, which still seems slightly out of reach for smartphones' limited "haptic feedback" (not that I saw that mentioned here, just something that came to mind when I saw this was an implementation for touchscreen devices).
Cameras, etc could be used to read our gestures:
Same with pinch-zoom or scroll. Or when working with a 3D model, making a hand shape as if you were gripping a globe, you should be able to twist and rotate objects on screen.
I remember posting excitedly about this on Slashdot 15 years ago and am just remembering it now. This should be easily achievable with modern libraries to get basic detection working.
Although some abhor the idea and want more complex keystrokes chords, for myself I think there are specific gestures I consider intuitive and wouldn't have to particularly learn anything new.
How does it compare to the Soli?