According to a Valve Engineer, the Steam Machine specs outclass the most common PC configurations. The company relied on its own hardware surveys to choose the system’s components. Even as an entry-level option, Valve claims that 4K/60 fps performance is realistic, despite a memory-limited GPU.
You can install Windows on it and should be able to play those games.
They are obviously saying that their hardware is powerful enough to play all games; not that all games support their choice of OS.