In this context, it means nothing. As you point out, GP is conflating pitch with pitchless phonemes, for example, the phrase "we say "Mmmmm" not something highe

In this context, it means nothing. As you point out, GP is conflating pitch with pitchless phonemes, for example, the phrase "we say "Mmmmm" not something higher pitched" doesn't make any sense. However, I suppose GP meant to write "we tend to say "Mmmm" with a specific, relatively low pitch", a statement with which I don't particularly agree. I would rather say it's done at an average conversational pitch, or even at a higher pitch since it often has a pitch accent conveying surprise. Which I believe is the main purpose of saying "mmm": communication, not vibrational tongue massage. Consider that when you say "mmm", you voluntarily choose a pitch intonation according to what you're trying to convey, ie. how pleasantly surprised you are.