Figma, worth $10billion, was migrating what seems like their core production data. They probably didn't want to bet the company on a comparatively small software vendor like Yugabyte.

Most likely the engineering cost was much much higher than your quotes, but still insignificant compared to the potential risks. And migrating from RDS to not-RDS could easily not have been cheap in engineering time either, depending on how much software they've built around it.