Because the Uniti Core also can act as a file sharing server that can be accessed by other computers over the network, like a PC or, well, a Nucleus.
And your screenshot looks like it is being accessed via an IP address, i.e., over the network:
In which case you would be using the Nucleus as the Roon Core server, while the actual music storage would seem to be the Uniti Core.
Which is not a bad idea as such - this way, you have one single storage for both the Naim system and Roon. And if you rip CDs with the Uniti Core, they are automatically also imported into the Roon library.
However, I am not a fan of setting up rather complex systems for other people and then leaving them in the dark about what was done and how it works. But that’s between you and the person(s) who did this.
The other potential problem with using the Uniti Core as storage for Roon is the one you already found …
Not quite. The Uniti Core can rip to WAV or to FLAC, both is an option but I think the default is WAV (unfortunately).
When using WAV, they don’t tag the files but keep the metadata separate in a kind of database, which is proprietary but, as you say, some software might be able to translate it. The WAV files as such are standard, but without any file tags Roon has no chance to find out what is what.
If using FLAC as the format to rip to, the Uniti Core uses normal file tags as far as I know. Unfortunately, it can’t translate the WAV files to FLAC after they are created. You have to choose this before ripping.
I own a Naim system but never a Uniti Core, so I think the above is correct but all of this is just what I remember from the Naim forum. You may be better off asking there for advice with SongKong and the tagging, it’s common problem over there. Here’s a search result from the Naim forum for threads about this:
Send greetings from @suedkiez if you ask about this on the Naim forum, I have the same name there but am not visiting much these days. 
Now, based on the IP address in your screenshot and if I am not mistaken, I still think the Uniti Core is being accessed as a file server over the network. Theoretically, this means that you can mount individual folders (HQ, LQ, etc, but also subfolders below that) as individual storage locations in Roon > Settings > Storage. If one of these folders contain your CD rips and nothing else, then this would be a way to identify them in Roon, because individual storage locations show up in Roon’s Focus and then you can choose this selection and save it as a bookmark or select all albums in it and tag them in Roon in one go as “CD”.