I think that Disc brakes are actually more useful on a TT bike than a road bike. In my experience, aero brakes on TT bike are pretty poor, while calipers on a road bike are very good. My latest build with a set of SRAM AXS rim brakes has stunning stopping power, excellent modulation and they don’t shriek like discs.
I am a triathlete, meaning I race whatever the weather. I want my TT bike to stop, even when I am riding full speed, in the rain with deep rim carbon wheels. If its raining and I am on my road bike, I am probably taking it a little more cautiously, or riding a bike more suited to the rain