This is true. The version that runs on Nucleus is customized for Nucleus, just like the version of RoonOS that runs on Merging’s NADAC PLAYER is customized for that hardware and ROCK is customized for suitability with a range of available NUC platforms. Those are the three RoonOS builds out in the world right now.
Some of the stuff that is different–the Nucleus Roon OS build has some thermal management stuff to account for it being fanless. It removes some audio devices in Settings->Audio so that ports not exposed on the hardware (like the analog outputs on the NUC inside) aren’t listed. It also identifies itself as a Nucleus, shows Nucleus branding on the configuration screens, etc.
The above is pretty basic stuff, mostly focused on branding, hardware differences that must be accounted for, and delivering a polished product. The implication above is that there is some extra secret sauce for sound quality in the Nucleus’s RoonOS build, but not in the other builds…and that’s not true. If things sound different, it’s probably because of some other difference.