That’s because the Nucleus is a niche product and it doesn’t really matter which kind of device is abstracted away behind an SMB network interface, as long as it speaks the SMB protocol.
The Roon server sends the music data out either via the network or by USB. If you use the network method, there needs to be something on the other end that can receive it. Preferably this is a streamer that is “Roon Ready”.
If you use USB, it can be simply a DAC with USB input. Preferably “Roon Tested” but it’s not completely necessary.
To a degree. That’s because barely anyone will try to market a streamer that can only work with Roon. Usually they will have a bunch of methods to stream built in, and Roon is one of the options.
Other options may be Tidal Connect, Spotify Connect, UPnP, AirPlay, Chromecast, or the device manufacturer’s own app. But none of them give you what Roon does.