What exactly does Close Loop Speed Control System control?
What it is actually controlling is the duty cycle, but the phase switching/commutation frequency follows the RPM so it will increases as the RPM increases since it must be synchronized with rotor or the motor will lock up.
It is wrong to think that the driver directly controls the commutation frequency. It does increase but the driver doesn't directly control it by dialing in a number. It just commutates whenever the rotor reaches particular angular positions so in reality the RPM (or more accurately how frequently the rotor passes a commutation point) controls the commutation switching frequency. And RPM is affected by both applied load and duty cycle.