Chapman Stick player here! The Harpejji has a very different tuning and technique. The melody side of the stick is tuned in fourths (like a guitar), and the bass side is tuned in fifths (like a cello). The Harpejji is tuned in whole steps, meaning the strings are much closer in pitch than they are on the stick.

The implication is that the technique used for the Harpejji is much more similar to piano: every note in a scale is played on a different string, and chords are produced by 'skipping' notes / strings. Whereas scales on the Stick are usually laid out using 3 notes per string on the melody side (exactly like the guitar), and 4 notes per string on the bass.

If I understand right, the Harpejji uses individual per-string pickups and uses an electronic muting system. Touching a string to any of the frets completes a circuit that causes the onboard electronics to unmute that particular string.

That's pretty unique.

The Chapman Stick just uses a mechanical damper where the nut is so the open strings don't ring out, and fairly standard magnetic pickups.

The commonality is "string instrument with many strings", which would include a lot of things (electric hammered dulcimers for example).

This, however, has important differences in design and operation, the most significant of which is the use of "an electronic muting system to dampen unfretted strings and minimize the impact of sympathetic vibrations."

I searched and only saw a design patent. The standard for these is much lower.