While I agree that that argument is very uninformed and misguided, I have to disagree with your assertion that it's because there's only 12 notes that pop music all sounds so similar. That's somewhat akin to saying "there's only 26 letters in the alphabet, so every single pattern has been done before."

While there's lots of redundancy and repetition of harmonic and melodic motifs in pop music, it has little to do with "only having 12 notes." There are psychological, mathematical, and cultural/historical factors that led to the Western music system and compositions derived from it being structured the way that they are. I've included a few sources to learn more about this, if you're interested. It really is a fascinating topic.

Tramo, M. J., Cariani, P. A., Delgutte, B., & Braida, L. D. (2006). Neurobiological Foundations for the Theory of Harmony in Western Tonal Music.

Gill, K. Z., & Purves, D. (2009). A Biological Rationale for Musical Scales. PLoS ONE.

Benson, D. J. (2013). Music: a mathematical offering. Cambridge: Cambridge Univ. Press.