Those Decks do not use PWM! Six outputs used, with those resistor values... it's a cheap-ass DAC
The main point is, without using PWM, you need some active buffer. In the Deck circuit, those six diodes and resistors could be replaced by a simple pot, and the transistors serve as a buffer between it and the LEDs.
Even if you do use PWM, you still need a high current output which a typical microcontroller cannot provide. So again you'd need a buffer or to use some dedicated chip.