To Valve’s credit, since that post they did implement base station power management and some DEs now implement Wayland’s DRM leasing protocol, and there’s a somewhat buggy async reproduction implementation in place (although it’s broken in SteamVR 2.0 onwards).
My assumption would be that it’s because we don’t really look at mirrors per se but rather the reflection in them, so the definite article is indicating the fungibility of the mirror itself. This total speculation on my part though and I might be totally wrong.