Nothing you've said here demonstrates any flaw in wintermute's logic, only that you've failed to grasp his point.
His point is that Mackey Rose's reasoning is tautological. That is, it's true by definition. If you judge a player's draft stock by where he's ultimately selected, then no player's draft stock could be significantly higher -- or lower -- than the number they're selected at.
We could apply this same reasoning to Perry Jones, III, and conclude that his stock was even lower than Plumlee's. After all, he was drafted after Plumlee. If teams were that high on him, he wouldn't have lasted until pick 28.
Mackey Rose's attempted point failed in another way...
Theoretically, any single-pick team, 1-25, could've had Plumlee as high as #2 on their big board (and two-pick teams, as high as #3, etc.), but as long as their top selection was available, would've passed on him.
For example, New Orleans' top three could've been Anthony Davis, Austin Rivers, and Miles Plumlee, but, since Davis and Rivers were still available when the Hornets were on the clock, they were the selections at #1 and #10. I don't believe this, obviously, but you get the point.
Anyway, all of this is just rhetoric. All I know is that, according to many reports, Plumlee was so impressive in workouts that multiple teams picking as high as the early 20's were giving him serious consideration. Knowing this, I don't see how anyone can say the Pacers reached for him at #26. You may not like the pick personally, and certain mock drafts may have been misinformed, but the 20's to 30's is where his draft stock had risen to, apparently.