Bottom line, the electric clutch/transmission concept would seem to be an advantage only when applied to the various multi-rotor (or propeller) designs. You wouldn’t gain any efficiency or design advantage with a single propeller aircraft of conventional design where, as noted, the objective is simply to couple the shaft of an engine running at optimum RPM to the prop.
There are, of course, other considerations. I think back to a non-aviation military system I worked with that was powered by a portable turbine generator. Even in the associated shelter the noise prompted people to start using earplugs, and conversation outside was nearly impossible. Translate this to a bunch of multi-rotor “urban mobility” craft…gulp!