It's correct. "XM Ready" means your head unit can control the Honda XM receiver, and is prewired for it, but doesn't include the XM reciever or XM antenna unless you pay for it (and the installation).
The black plastic cap on the rear roof is just that...a black plastic cap. It's not the XM antenna.
And I'm not sure if the XM receiver, antenna, and mounting kits are even available anymore. I checked a couple of Honda accessory websites and don't see them listed.