The advice wasn't directed at me specifically - it's given for all prostate RT patients here. They said it's because the area will be inflamed, and cycling on it will make side effects worse.
Did a 10 mile leisurely ride yesterday shortly after RT on the new saddle, and it felt absolutely fine - no pressure on perineum whatsoever. I am noticeably lower energy levels - that's probably down to the RT which is known to have that effect.