Software similarity and market positioning don't really come into consideration once the role of gatekeeper has been established.
They had valid reasons for it.
There are no rules that are governing what is or isn't a gatekeeper. It's just whatever EC decides that day.
It's more complicated than this: the EC has the initiative for legislation in the EU but the text they submit is later amended and voted by both the European Parliament and the Council (representing member states) so it's not true that the EC defines the rules. And both the member states and the European Parliament are pretty jealous of their prerogatives in the decision process so you can be sure that the EC cannot have arbitrary power that bypasses the Council and the Parliament.