The occupied bandwidth of a signal depends on it's symbol rate (aka baud rate). Slower symbol rate results in a narrower bandwidth signal. The article mention

The occupied bandwidth of a signal depends on it's symbol rate (aka baud rate). Slower symbol rate results in a narrower bandwidth signal. The article mentions that this slowed down the baud rate of the serial port. So it sounds like the clock rate ends up slowing down everything that depends on it and the narrow RF is a side effect of that slower rate. Whatever generates the carrier frequency must be separate from the main system clock if the center frequency isn't effected.