And you can only check the boot volume (you can't repair the boot volume this way - you have to do something else like boot into single use mode or safe mode will do it automatically).

No, it means exactly what he said: You can't repair the startup volume. I can repair any volume (except the startup volume) that has Mac OS X installed. I've got a second disk that has the system installed on it and I can repair it from the startup disk with First Aid.