Obtaining the advantage?

Over the weekend I was reading Viacheslav Eingorn's Decision-Making at the Chessboard, having decided both that my decision-making process is entirely inadequate and that I can remedy that fault by reading a book about it. However, my late and abrupt flowering into masterhood was interrupted when I got distracted by wondering about the validity of a comment he made as early as page 9 (suggesting that concentration might be more prominent among my problems). Anyway, Eingorn is showing us the famous Reti-Alekhine game from Baden-Baden 1925 and in the course of this exposition he paraphrases Alekhine's comments on the game:

with 20...h5 Black started an interesting counter-attack and with 26...Re3 he obtained the advantage.

Reading this, the first thing that occurred to my perhaps over-pedantic mind was: how could Black have obtained the advantage? If after his 26th move he had the advantage, surely he had it before his 26th move as well? Isn't the nature of an advantage that it is inherent in a position? That provided moves are available to you which subsequent to their execution will give you a better position than your opponent, you already possess the advantage?

This being so, surely an advantage can only be provided by an opponent's error rather than by our moves as such, however good they may be? Provided we neglect the question of whether there is an advantage to either side in the opening position, which is in any case irrelevant to Eingorn's point (or Alekhine's, as paraphrased by Eingorn) I'd suggest that it is. Black must already have possessed the advantage before he moved. You cannot create your own advantage.