Then be astonished, I think.
The actual option under Squuezebox devices is “Use FLAC compression”. So, if you have DSP enabled, Roon is decompressing the FLAC applying DSP and then re-encoding to send to the Squeezebox. Not sure whether it does this if there is no DSP to be done, but, I wouldn’t be surprised if that was the workflow in that case as well.
Here is a post about this:
As for Linn and its own “protocol” solution, I have no idea and I don’t think Roon has every publicly stated; what it really is nor how it functions. (at least I couldn’t find such a discussion, nor, ever remember reading one).
So, no one really knows except Roon and Linn what Roon is sending the Linn devices. It could be PCM, but, if your friend is showing the original format that is being played by Roon, who knows? It might be sending PCM and that extra information to the Linn box for display purposes.