Flexibility in the forward bending position doesn’t all come from motion at one lower lumbar segment. The length (tightness) of the hamstrings is an important factor.
And when motion at one segment is decreased, the other spinal segments may compensate to make up the loss. This means that increased motion in the upper (thoracic) spine and even in the upper level of the lumbar spine helps make up the difference.
The fusion is considered a failure only if segmental movement is seen on flexion-extension X-rays.