if it isn't supported, then why is the option even there? It's very annoying to me to have something look like its an available option only to find out it doesn't work as expected.
The option is there only for TV tuners that
require connecting a cable from the tuner to the sound card. Those old TV tuners do not process sound internally. Instead they route the audio to the sound card. The result is that when TV is playing, the sound goes to the speakers directly, as long as the particular output channel on the sound card is not muted. The option in MC is provided so MC can mute that output line during time-shifting - otherwise you would get double sound - the original sound and the time-shifted sound.
Therefore, now that I have thought of it more clearly, your choice for "Choose Mixer Device" should be an output device, not an input device. So "Line In (High Definition Audio" may not have been the correct choice for you.
I know this is quite confusing, because it is old, there is another place where you may need to look at. While TV is playing, right-click to get a context menu, and go to "TV Options\Devices", and check the correct audio device.