No, it is part of the official distribution: openhab-addons/bundles/org.openhab.binding.millheat at main · openhab/openhab-addons · GitHub
It is no problem to support both cloud and direct gen3 in the same binding, just use a different thingID (ie gen3-heater or something). Most code can be moved to a separate subpackage (i.e. .cloud) to keep it as separate as possible from the direct connection code for V3 devices.
Or you can - as you suggest - create a separate binding. I’ll leave it to the addons maintainers to share their preference.