@xyzzy You know that the myths mentioned here are the same as myths from our world... Right?... The Greeks and Romans had "scholars and shit" who were far more advanced than those alluded to thus far in this story, yet Iris was still a goddess. The correlation between rainbows and fountains/rainshowers was also pretty common... But was explained away via those myths rather than magically understanding how white light consists of the full spectrum of visible light, which are differentiated by their wavelength, thus allowing them to be separated when passing through the interface of two materials with different indices of refraction at a certain angle.
"God is blessing us" is a much more reasonable conclusion to make when everyone believes in such a higher power and nobody understands optics.
Also, note how those who actually believed it was a bridge were all commoners and the boy who only saw it because he was skipping studying it. Nobody there understood how rainbows formed, but they were all willing to ask. You would probably have the same reaction if a kid brushed away some mystery of quantum mechanics like it was common knowledge as well, even though we do have "scholars and shit."