They're saying the server leaked its own IP address. Unless you've set up your system so that your Tor hidden server is on a computer not connected directly to the Internet and it connects to a physically-separate Tor node that blocks any network flows other than ones going over the Tor proxy, then any Tor hidden server also has a leakable IP address. A Web server error message (or embedded error message from a third-party service, for example), header, or other piece of data might then contain the server's IP address.
That's pretty thin information by itself. But if any part of your server is configured to listen on all network interfaces (instead of, say, localhost), then someone making an HTTP request to that IP address gets a page from your server. That's fairly damning evidence.