Correct open equals nothing. Going by the wiring diagram pin 1 of F has 12 volts at all times and will supply 12 volts to the ecm via pin 4 when the brakes are depressed. That would mean when the brake pedal is not being operated F gives no signal of any kind to the ecm. When I say I left it open I mean I did not put a wire in the terminal of the ecm.
You do not see the circuit correctly. F is the Brake Light circuit and it goes to the "hot" side of the brake light. The ECU in-turn sees "ground" thru the light bulb in the brake light when the light is off. Yes, it isn't a "perfect ground" as it is a couple of ohms or so, but it does act as a pull-down resistor to make the circuit low. Putting the brakes on, 12V is present, makes it high.
Diesel pedals are Diesel pedals. The 1x6 terminals ones, with DIESEL written on the side of them, have the same pinouts and they are interchangeable. The older B4/MK3 pedal with the 2x3 terminal on them work too, but the wiring is slightly different. Early 1998 Beetles had 1x6 pedals, then 2x3 wiring in the dash with an adapter in the middle to change the pins.