Your wording in your original post caught me off guard a little bit.
I joined ONLY to shame the hacker to his friend, but it appears another community now has the news, too and they seemed grateful for the information.
If you went there with the intention of shaming the hacker rather than deliver the information to another community, that's a little...backhanded, in my opinion. Then there's the conversation we could have "do the ends justify the means" or "does motive matter."
But let's say you didn't share that little tidbit. You went to another community, talked to the people on it, shared that he was hacking, and warned them to watch out for him. What they do with that information is up to them. If you went on the community and said "permaban this hacker, we just caught him hacking on our server!" that's another story, indeed, but if you just warned them and then they permabanned him, okay, fine.
That being said, I do agree with this.
Sometimes admins miss things. Sometimes on TTT I suspect people of ghosting and cannot find a connection, or have someone RDMing and don't realize they've been going server to server getting themselves banned for mass rdm. Sometimes a suspected hacker doesn't have any proof against him, so we cannot act on it, yet another community could have demos from earlier that day. While we could dig for previous bans for every ban we do like we do for accepted admins, having a list of SteamIDs or names could be helpful - not that we would ban them immediately, but in case we did suspect them of something, we could be ready to get proof.