The same old song

Can you analyse this? "You don't achieve harmony when everybody sings the same note."

I love music but know nothing about it. Merriam-Webster defines "harmony" as:

1 archaic : tuneful sound : MELODY
2 a : the combination of simultaneous musical notes in a chord b : the structure of music with respect to the composition and progression of chords c : the science of the structure, relation, and progression of chords

I am guessing that you would need various tones that harmonize to create harmony.

However, I can't imagine anyone using the phrase except figuratively. If everyone says exactly the same thing, it's not harmony, it's monotony. SS