The Help file is incorrect. The Mixdown command affects the selected audio data in all unmuted tracks. So if you don't wish to mix down all audio tracks in the file, you need to follow the three step process below:

1.

Mute all audio tracks you do not wish to be included in the mixdown.
2.

If you're mixing the entire duration of the track(s), no time selection needs to be made. If you only need a portion of the remaining unmuted tracks included in the mixdown, select that time range.
3.

Choose Tools | Mixdown Audio.