There doesn't appear to be robots.txt restrictions. I can get back results by doing a site:kbin.social search or a site:lemmy.ml search.
I think that a bigger issue is that the fediverse systems are designed to span multiple hosts. There's no convenient syntax to ask Google to search all content on a given fediverse platform, because it spans different domains. Reddit content is on reddit.com, so the site: operator is sufficient.