if i understand correctly the exercise suggested, the stated leg lift exercise will:
actually exercise the quads isometrically (involves no movement or contracture/shortening of the muscle(s) involved), & in part involves the knee joint, although statically,
will exercise the hip flexors isotonically (involves movement, or contracture/shortening of the muscle(s) involved - and, upon the "return" motion lengthening of the muscle(s) involved).
[Note: contracture, as used here, refs to just the concentric portion of a contraction when the muscle shortens against the load and not the eccentric portion of the contraction when the muscle lengthens against the load - e.g. when returning to the starting position of the rep.]