That's an old tale from about 5 years ago, debunked here. "Rainbows are not dependent on the shape of the Earth. For example, there are rainbows on Titan formed by water, and rainbows formed from methane droplets.
What matters in forming rainbows is water vapor (mainly, and Earth has lots of this, unlike most other celestial locations we know of), sunlight coming from behind the observer, and an observer. Rainbows can form with other suitable liquids that from droplets and have a refractive index in the appropriate range, e.g., methane droplets, but we get lots of them here on Earth because we have lots of water, and it is often in droplet form in the atmosphere."
