St Giles, you miss the point. Before the Flood, the amount of moisture in the air was (arguably) far, far less than today. Because, before the Flood, there was no rain. There was no evaporation, to the degree we have today. The entire moisture/rain/evaporation system which exists today didn’t exist pre-Flood.
Ergo, if moisture levels were much, much lower and if it never rained, how would a rainbow happen? How would anyone see it?