I assume that they mean that the identity of the people running the Tor relays can be known. For example, if you run a relay from your own physical server, then your ISP knows who pays for that static IP. If you run it from a VPS or cloud, the company knows who's paying for that server.
But if you live in a liberal democracy, none of that should be an issue as far as I know, specially if you're running a non-exit node.