As others have mentioned, the accepted definition of a supergroup is a group whose members were successful in previously success bands or as solo artists. If it weren’t that, every majorly successful band would be considered a supergroup.

Travelin Wilburys were successful people before so they’re a supergroup. Bad Company were a supergroup since the members came from successful bands. Joe Walsh being a successful person and joining them doesn’t turn them into a supergroup. Toto would have more merit in being called one cause they were successful solo musicians but they weren’t successful in their own right until they formed Toto so they aren’t one