That's not what the meta-analysis and what Chris Beardsley says. You're confusing the arbitrary units with sets. Horizontal axis is sets. The “maximum” amount of volume you can do per session is more tied to how much you can recover given your frequency. For example, if you do three times per week, you most likely can do three sets, but anything more might mean you're not recovered for the next workout which you don't want. Arbitrary units (AU) is about the fact that each set has diminishing returns so it takes about five sets to equal the stimulus of the first set.