Long before that, Optikey has had "multi-key selection" which works in the same way, see https://youtu.be/HLkyORh7vKk?t=10

In my experience (as a developer of eye gaze interfaces, not an everyday end user) it is often more efficient to have really good next-word-prediction (such as with the Presage engine) combined with single-letter dwells, rather than using "swipe-like" spelling, where you're committed to tracing out the whole word.

Combined with predictive typing, and you could go pretty far.