Not necessarily.

There's a video somewhere on Youtube (and elsewhere), IIRC, it was made by a German to demonstrate how much an overweight person could conceal just in rolls of fat. It was a very effective demonstration of the limitations of the NoS in scanning very obese people. IIRC, he was, in part, countering the arguments of some that the surrender position would 'stretch' fat rolls and raise breasts so far that anything concealed under them would fall out or be exposed.

Or is TSA suggesting a 'part 2' scan where the pax bends over, spreads legs and grabs ankles, to dislodge/expose anything concealed between his/her cheeks?

I've seen many individuals, of both genders, including quite a few in blue uniforms, who could have successfully concealed fairly sizable items between their butt cheeks (or in their fat rolls).

Of course, if it's metallic, the WTMD would probably catch...but WTMD is no longer primary...