You quoted too selectively.
"A well designed system is balanced: when the input load increases, all parts of the system take the hit."
"Given this reality, the processing power of the application cluster is arguably the most important factor."
It's just nonsense anyway your look at it, really.
"if you need to use 100 mysql servers, you better darned use 100 computing servers of equal computing power, because if you only use 50 you're not balanced."
The second does not follow from the first.