he hopes you're not a beginner before delving into queens gambit theory ...
The passage refers to "hoping" that the reader has "improved enough" for "this opening". That does not seem to me to be a general instruction that a beginner should read no opening theory. By the way, did dfgh123 ever produce a specific sentence where Capablanca "says opening books are for experts"? How about a specific sentence, saying that part 2 is not for beginners?
capablancas chess primer is split into 3 parts
part 1 is for beginners (no opening theory)
part 2 is for average players (some opening theory with a suggestion to learn half a dozen openings)
part 3 is for good average players (complete master games to study)
Let's look at some actual quotes.
"... 'Chess Fundamentals' ... does not deal so minutely as this book will with the things that beginners need to know. ..."
Notice that it says, "this book", and NOT, "part 1".
"Part three is not for beginners. By the time the student has reached this part of the book, however, he should be a good average player."
I see nothing about the reader being supposed to have become an average player by the end of part 1. Pages 83-114 (from part 2) are about endgames, including such topics as "passed pawn", "opposition", and bishop-and-wrong-color-rook-pawn.
More stuff from part 2:
"... you may be behind in all three of the other elements, Material, Space, and Time, and yet have a winning position. ..."
"... the centre squares ... are the most valuable squares both in the opening and in the middle game, and often also in the end game. ..."
"It is now time for the reader to familiarize himself with a number of 'Type' positions and combinations that are apt to present themselves during the middle game. ..."