Do you have a source on that? In the Weather Feature Discovery video they said they’re using a scattering model to accurately compute the angles and visibility conditions for rainbows. However, the rainbows themselves are probably still “painted” on the screen when certain conditions are met. A realistic model handling the dispersion, refraction, and reflection of an actual rainbow is extremely complex. That’s tough to do and beyond the scope of even many dedicated ray tracers, let alone rasterized game graphics like Flight Simulator that “fake” a lot of effects through simplified or precomputed shaders.
If there’s too many rainbows in the sim, it’s probably for the same reason there are too many red sunrises and sunsets in the sim. The scattering model spitting out these effects is always “on” and many times IRL these optics are blocked by layers of cloud and atmosphere.