Forgive me if I am missing something here (I do not get involved in much surround work) but it looks likes Mix6to2 is a down mixing - 5.1 to stereo - plug in. If the result is a 5.1 MP3 why are using a downmixing plug in in the signal path ?
Additionally if you are expecting a 5.1 multi channel audio file to be output why are you selecting
stereo for the rendered file type in the export dialogue ?
If you select stereo and use a downmixing plug in in the ginal path (mix6to2) the logical result will be a down mixed stereo mp3 file.
You may need to more clearly explain what you expect as the output file format/purpose
and follow that through the “signal chain to export file type” in a logical manner.
Just looking at the article the critical bit is probably : “select the 5.1 output as the audio engine output” Mix6to2 is not mentioned.