An akrasia strong enough to defeat GTD or Pomodoro techniques may not be strong enough to defeat a guy you've paid a lot staring at you and waiting for you to do the next easy step. The people who would feel the need to go to such workshops are a self-selected group.

This is very true, but the simpler explanation for GTD/Pomodoro failure (or any other technique failure) is simply that it isn't addressing the right part of the problem. If you don't want to be focused (as opposed to "wanting to want to"), a focusing technique simply isn't going to help.

IOW, although self-selection occurs, postulating strengths of akrasia or meta-akrasia is an unnecessary hypothesis.

In particular, it emphasizes the idea that akrasia is a thing, when in fact it is a conflict between things. Conceptualizing akrasia as a real thing is both an epistemic error and an instrumental inefficiency. You will treat it as a thing to be fought or cured, rather than as an emergent property of conflict.

Heck, even the name, meaning "failure of will" is wrong. It directs one's attention to willpower-based solutions, instead of focusing on that which causes you to need "will" in the first place.