It seems to me to be the same logic which requires LSF cable in case there's a fire because it doesn't emit noxious fumes; what do you run that in? PVC trunking.
Or the same logic which requires fire rated P clips for FP cable to stop it falling down and obstructing firefighters, but the other cables are somehow fine held together in a bunch with a nylon cable tie.
If anything i would have thought it would make more sense to use an isolator if there IS a window because the window would reduce the need for forced ventilation, so you could happily switch the fan off if for instance you wanted to go for a pee in the night without the fan waking everyone else in the house.