It is impressive in that the model just does it by itself when the conditions are present.
Like you say, if it surprised the developers, then it’s not just an “If clouds are present and the sun is…, then show a rainbow.” It sounds like it’s more similar to surprise reflections from ray tracing reflective surfaces. Kind of like in tons of photographs where you can see the cameraman in the sunglass lenses of the subject.
That is pretty cool.