I’d say that Hemingway has a transparent style. No hidden agenda or technique, just simply laid out sentences, one after the other, and it certainly is not Young Adult.
The comment about being transparent strikes me as more condescending than constructive. I bet you’re kicking yourself for not asking, “what exactly do you mean by that? Can you give me an example?”
The question is: do you think the book is young adult? Do you want it to be? What do others think?