on paper that may be true, but in reality, it's not.
many electronically commutated (brushless) motors don't like PWM at all, and all though you might not hear the pwm itself, you get buzzing and growling in addition to poor fan performance.
sometimes fine-tuning the duty-cycle and frequecy while the fan is running helps to minimize this, but often times you're left without the option of freq.
the fan in my gaming machine makes a loud growl at 55% duty, but is much quieter at 51%, I've got no idea what the freq is, I guess whatever AMD and DFI came up with.