That’s what they said, but the rainbows definitely appear to be “painted on effects or textures” that are activated when certain conditions are met, rather than an emergent phenomena of the sim’s model of light and water, or the result of dispersion and refraction calculations.
For example, if you drop the sun angle, the atmospheric scattering model correctly colors the light red (because the blue light has been scattered away). The rainbows, however, are unaffected, and still show the full spectrum evenly (rather than red and yellow being brighter and blue and violet pretty much gone).
It’s cool that they took the time to implement them, getting the angles and visibility conditions correct. And I think this is what they meant in the trailer video. It’s not just a fixed thing that turns on and off, but is a dynamic object. They always seemed a little fakey looking to me though, like they’re too “flat”, like a painted texture map rather than glowing with natural light.