It's the 100% pairing, or worse, 100% mob programming (up to six people with one screen!) that I can't believe would ever be more efficient than the programmers working independently.

Sure, have a party and mob-program for fun for an afternoon, or solve a particularly sticky problem that way, or use it to transfer knowledge about the system to the wider team. Have a blast, and teach everyone something useful at the same time. By all means.

Just don't pretend (as pairing/mobbing advocates do; not you) that two people sitting at a computer can be as efficient at day-to-day coding as two people coding separately, unless at least one of the two is a junior or near-incompetent developer who would contribute negative productivity on their own.