That's 100% right.
Nikon lenses are designed to stand further away from the sensor/film than Canon lenses. So you can use a simple mechanical adapter to fit a Nikon lens on to a Canon body, and if the thickness is correct then it will work fine. But the other way around, any kind of adapter on a Nikon body is going to push the lens further away, whereas a Canon lens needs to be closer. So the end result is that you lose the ability to focus on distant objects. Or in principle you could get an adapter with corrective optics, which would be expensive and would degrade the image.