Aagard has a decent-to-good reputation--I think I got through one of his books, or at least most of it--but without a definition of amateur (which may be in the book, I don't know) he appears to have a problem with boundaries.
Also, IMO there is an apples v bluefish in the title. To me, an amateur is someone who plays but doesn't make money in the endeavor, and a professional is someone who does (and relies on that income as life-sustaining revenue). The GM that comes to mind that could not be considered a professional in Zvjaginsev; one of his games is in the Immortals section. He is clearly outlandishly strong but simply doesn't play enough to support an assertion that it's his occupation.
I would take Aagard with a grain of salt in this case. Hyperbole sells. Calling a 2581 rating an "amateur" is just silly. Byrne called Fischer "just a kid who played a good game against me" regarding The Game of the Century. I wouldn't get worked up about this book, and Aagard's cred just took a hit given your post.
I do have a book review forum in the EZS, would be grateful for a contribution since it's fresh.
I read Grandmaster versus Amateur (Quality Chess 2011, not the book by Euwe), and I just discover that GMs don´t think of us as amateurs. So, right now, I don´t even know to which category I belong.
Aagard includes a game against Alan Grant rated 2200, for example about him he says: "Alan is rated about 2200, but he is clever and resourceful player", well, at 2200 he should be glad to at least be clever.
He also includes a game with Angelo Damia who just was getting his third IM norm in that game. So then I realized than even IMs are considered amateurs.
Peter Heine Nielsen includes a game against Magnus Carlsen(!) from 2005, he says: "by the time of this game Magnus´s rating had already shot up to 2581... casting him in the role of amateur certainly stretches the definition of the term, but despite his phenomenal abilities he was still only a young kid who had a number of holes in his game".
If somebody rated 2581 has a number of holes in his game, what can be said about the rest?
The book continues like that, it´s not really about GMs beating 1600 - 2000 players, is about GMs beating FMs, IMs or young GMs.