No no, that's not a rule Steam has. Their rules are just referring to overlap and certain things such as directly linking your pages and game file downloads together, because of different policies. You can, for instance, publish a game on Steam, Itchio, GOG, and a personal website all at the same time, there's lots of games that do that, so long as you follow each site's rules about how you handle linking between hostings. For instance, Mouthwashing got de-listed off Itchio because it's page just linked you over to Steam, which is against the rules.