Here's some notes from Bhikkhu Bodhi and the Commentary (Spk) that Thanissaro Bhikhu comments on in the first post. Which does indeed seem rather convoluted.
"Bhikkhus, I will teach you the all."
Spk: The all (sabba) is fourfold:
(i) the all-inclusive all (sabbasabba), i.e. everything knowable, all of thiwhc comes into the range of the Buddha's knowledge of omniscience;
(ii) the all of the sense bases (ayatanasabba), i.e., the phenomena of the four plandes;
(iii) the all of personal identidy (sakkayasabba), i.e., the phenomena of the three planes; and
(iv) the partial all (padesasabba), i.e. the five physical sense objects. Each of these, from (i) to (iv), has a successively narrower range than it's predecessor. In this sutta the all of the sense bases is intended.
The four planes are the three mundane planes (sensuous, form, formless) and teh supramundane plane (the four paths, their fruits, and Nibbana).
"If anyone, bhikkhus, should speak thus: 'Having rejected this all, I shall make known another all' --- that would be a mere empty boast on his part."
Spk: It would be just a mere utterance. But if one passes over the twelve sense bases, one cannot point out any real phenomenon.
"If he were questioned he could not be able to reply and, further, he would meet with vexation. For what reason? Because, Bhikkhus, that would not be within his domain."
Spk: People become vexed when they go outside their domain. just as it is outside one's domain to cross a deep body of water while carrying a stone palace on one's head, or to drag the sun and moon off their course, and one would only meet with vexation if one makes the attempt, so too in this case.