Hi Loic,
I agree lens center is an unexpected name for the focus of the lens. Here, you can replace the term lens center with pinhole or focus of the lens, and everything should be fine.
The next image depicts how it works in reality, you can see the lens center is the same point as the focal point. In modern cameras, there is a pinhole + lens, and the lens is put in such a way the that lens center (focus) and pinhole are at the same point.
