A lens aperture will be held open to its widest aperture for focusing and then stopped down just before shutter release. So, if you have a f/2.8 lens and taken an image at f/16 the focus system will use the f/2.8 for focusing and then stop down to f/16 for the shot.
Now, if you have a lens with a smaller maximum aperture, say 5.6, or if you use an f/4 lens with a 2x teleconverter which would give f/8 then the autofocus system has to operate with less light. The spec is saying that a certain subset of the autofocus sensors are rated to operate with lenses that have a maximum aperture up to f/8. Many previous cameras were rated up to f/5.6. Of course, ambient light conditions will have an impact.