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The motivation given was "Not an example. There's still New Game+, after all."
I contest this — they may be useful in New Game+, yes, but, for the main game, they do count. Furthermore, I'd classify that sort of thing as more of a subversion of the trope.
(And even so, one can still say that the Obsidian Skull gotten from beating the final boss on extreme and the post-NG+ books count just as much.)
The NG+ adds enough content that it can be considered part of the main game. As to books, I got and equipped Azra with some of the books before finishing the first game, and some of the NG+ books before finishing the NG+. Your point is valid, but it's debatable whether these are true examples.
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How well does it match the trope?