All seat belts and even isofix have a ‘bit’ of give in them, so that the full impact force is not transmitted directly into the body. Even a tiny movement can significantly reduce impact forces.
It’s why rock climbers use dynamic (stretchy) climbing ropes as opposed to static lines. A big fall on a static rope could break your back.
My understanding of isofix is that it is not necessarily safer than a correctly-fitted seatbelt child seat, but the difference is that it is much easier to fit a conventional child seat incorrectly.