Yes, that's correct. All PC/XT keyboards have the same connector as the AT. But they are not compatible with AT or other modern PC's. The PC/XT boards only sent data to the computer, but did not receive data. With the introduction of the AT, IBM realized they needed two-way communications in order to add LED lights for Caps Lock, etc. In doing so, they modified the protocol enough so that XT class keyboards are incompatible with later AT and above keyboards. A few manufacturers added a switch to change between the XT and AT protocols, but IBM wasn't one of them.
In other words, these keyboards are worthless for use on a modern PC unless you make an adapter for them. Even then, the layout is unacceptable to most. It may look a lot like an AT, but the differences in layout are enough to drive most to not use the XT, even if they could.