This is a reply from their social media team that i got back in September when they released summer 2021
"Right now we cannot see how we would be able to operate to Cardiff from anywhere in our network in a way that provides a schedule suitable for customers and financially viable for us as an airline,"
This reply seems to back up what the CEO himself said in an interview with Anna.aero and on a LinkedIn in post who also mentioned Easyjet adding extra flights at Bristol and in the interview said some airports were more helpful than others.
Maybe they will return in the future but I'd suggest they'd need more aircraft to do so and I can't see it being in 2021. Will be odd not seeing Edinburgh as a route from Cardiff.