Not a virtual machine. It’s just that Roon OS has swapping RAM to disk turned off. Therefore, either there is enough RAM and all is ok, or there is not enough RAM and then it crashes. If there is more RAM than „enough“, the additional RAM is unused and does nothing.
(In contrast, general-purpose operating systems can swap RAM content to disk if there is not enough RAM for the running tasks. This avoids crashing in unpredictable interactive environments as you can’t predict how much stuff the user will start, but it can make performance somewhat unpredictable if there is RAM pressure and stuff gets swapped to disk.)