paramount: <Rama: Very nice. Surely the game would've made it into My System's (or Praxis') collection if he had.>
Actually, My System had put this game in its chapter 14 "Overprotection".
In the book Nimzo wrote about move 20.Na3 "Only now will it be clear why white delayed with the development of the Knight. An honorable post had been contemplated for him, namely as overprotector of the d4 pawn. "
I mean, from all the good theories about chess in his book My System, the part about Overprotection is the most vague, i say that its not entirely useless, sometimes we have to over protect our pawns (particularly the base pawn or the isolated pawn) but that depends altogether with the condition.
I mean that Nimzo deliberately delayed the knight to Na4 then to Nc2 from the onset just to overprotect d4 pawn, i doubt it.
My System is a classic chess book, its brilliant on its era that provides the important theories like rook on the 7th rank, the power of the isolated d-pawn, pawn structures, and many more.
And this book still a good reference for the beginners/intermediate now just to learn its outline theories about chess. BUT, if you want to go deep studying the games from the book, prepare your ENGINE, because 90% of the games examples of the book contains some weaker lines even some are absolutely inconsequential. So you will completely need assist to learn examples from My System.
I suggest you learn this book first before go to the deeper/harder books like Dvoretsky's School Chess of Excellent/Manual Endgame.