Yeah, the differences are pretty much the things listed under “limitations” in that document. The existing option does a virtual three-way using Z-wave association, and has its drawbacks, as listed. For me those are delay and issues with two-way sync; I’m going to do this anyways, and I think I have a way to handle the two-way sync.
The proposed feature would turn an Inovelli into a “companion”, rather than a smart switch, at least for electrical purposes. The “companion” Inovelli would communicate with the “live” one using the same signalling that the GE and HomeSeer companions, down the existing 14/3 cable. This would have the advantages of instantaneous communication and avoiding some of the complexity and debugging needed with Z-wave association. If the “companion” parameter could be set from local config mode on the switch, then it could be set up even without a hub.
Of course the “companion” could still join the Z-wave network if desired, act as a repeater, and (if it is Red Series) send Central Scene commands when buttons are pressed. But none of this would be necessary, because the wired three-way would just work.
The unknown for me is the details of how the companion signalling protocol works, so I’m not sure how difficult it would be for the switch to implement it purely in firmware without hardware changes (which probably wouldn’t be worth it).