Ondrej was talking about self-hosted forums, where someone creates the forum and also has access to the server. So even if it was possible for another admin to remove their admin status, they could create a new admin account.
You didn’t mention you were talking about a forum hosted by Discourse. I don’t think Discourse ensures that the first admin isn’t removed on their hosting. I can imagine it’s not that rare for someone to create a forum, leave a company later, and then the first admin needs to be replaced, so it makes sense to me to allow that.
I guess, in the worst case, if you accidentally grant admin permissions and confirm it via email, you could contact their support, and they could check who created the forum.