I think that's a bad analogy. The rectus abdominis is considered a single muscle since it is ecased within a single sheath, but unlike the biceps, the rectus abdominis is obviously segmented. The lower segments are comprised of entirely different fibers than the upper segments.
Additionally, the RA has something like eight different sources of innervation. While the tension at both ends of the muscle is going to be equal during a contraction, I don't believe that anyone has ever shown that the different segments aren't recruited to different degrees.
So, while I can't claim for fact that there is no meaningful distinction between upper and lower abdominals, I don't think it is a completely faulty concept.