Well - he does not mean mono in the sense of mono-stereo…
When he says mono, he means monophonic… The things he does do not work very wel with polyphonic material (more than one note at a time - in the sense of several instuments at the same time). Even a mono (thus one channel) signal wil not very well if the material is polyphonic. At the other hand - a stereo (two channel) signal will work nice if the source is monophonic (thus single noted, or only one instrument) - just as demonstrated…
Think of the chord track as an automatic version of pitch correction. You can use pitch correction to create a harmony to a monophonic instrument or transpose it. However you cannot pitch correct or transpose a harmony instrument that plays 2 or more notes simultaneously because the software cannot distinguish between the two or more notes. It can only read one note at a time.