I don't know if we disagree. My only point is that if you run the U-lock as pictured, you cannot remove the bike without cutting through either the lock, the rear wheel, or the frame. The one thing you cannot do, in other words, is carry off bike-sans-rear-wheel by merely disengaging that wheel and leaving only it behind (which is "topologically impossible"). Sheldon Brown's point was that a thief, when confronted with this configuration, and who lacks the means of cutting the lock itself (otherwise you're doomed no matter how you locked it), will be forced to cut either the frame or the rear wheel. If he does the former he ruins the frame; if he aims to do the latter, he's confronted with a difficult and time-consuming job that most likely would not repay the risk and hassle.
I think annc above had it right. If there is a reason to involve the frame, it is for a display of deterrence: i.e., to deter a would-be thief who doesn't understand all this and who might try yanking the bike around after he's disengaged the rear wheel.