Umm. If Roon Server is running as an application within an OS, it is benefiting from the underlying OS’s management of the hardware (CPU, RAM, etc), which Windows and MacOS is able to undertake.
There are many favours of Linux and different modules within the kernel. We have no insight into what is used and how RoonOS works as part of ROCK - apart from the comments made about its single thread per zone operation.
I did experience this when pushing a NUC5i3MYBE NUC to the max, as a 4-core CPU, a got playback with DSP (upconverting to max PCM, converting to DSD, EQ filters) all playing back on this entry-level NUC without issue.