No, the camera is indeed allowed to change a tiny bit. For example, you do not need a tripod. Taking photos with a handheld camera works fine (although a tripod works even better). They explain it in greater detail in their paper: https://arxiv.org/pdf/2004.00626.pdf

Background subtraction methods on the other hand usually fail if the camera moves even a tiny bit or the lighting changes slightly. More advanced methods can recover eventually, but you still get a few frames with improperly removed background.