Far be it from me to dispute with the renown commentator. I just don't think it makes any sense to narrow Paul's rejoinder to a naked assertion of authority, of pure power. And I would like to see the actual logical dissection of the (proposed) argument, because the questioner still looks to me like he's offering simply a false (but common enough, even today) inference from the previous defense of salvation stemming from sovereign election. Here's the knee-jerk reaction to the jarring realization that election entails particular grace; individual destinies are foreordained prior to any human act. The heart that has not been rendered docile by grace instinctively rears in rebellion to such a display.
If God is righteous, then he's fair almost by definition. The accuser thus directly impugns divine righteousness, a major concept that permeates the letter. Paul reasserts that righteousness in the first words (v20) of his reply, "Who are you to talk back to God?". God's right (v21, exousia) is righteous; I think the next statement follows from the previous. And following that, he gives a reasonable proposal for why God might ordain matters as he did ordain them: because he finds it suitable to demonstrate both mercy and wrath, vv22-23, and neither of those determinations are more ultimate than the other.
It seems to me, the hypothetical questioner admits that God has undeniable and unopposable power (...for who resists his will?). And maybe the "equivocation on will" is comes in here, because divine ability and authority are perhaps being improperly divided? I'm not quite seeing it that way, again unless right and righteousness are acknowledged as essentially indivisible, or two sides of one coin.
Maybe "false inference" is what you're looking for. But I don't think that Paul so much deconstructs the questioner's error, as he rebukes the pride/rebellion that spoke. I have tried to restate Paul's assertive counter-proposal, which certainly includes reasons.