Is the main character identical to 21st century humans? Where did the life on the planet come from, and where did the life on our planet come from? Just because the author doesn't spell out everything to you it doesn't mean that the situation is as straightforward and low tech as it seems.
Of course, circumstances can vary. For example, in The Word for World Is Forest, there are great woods of the same trees growing on another planet as on Earth. But that book is part of the Hainish cycle, and virtually all the inhabited worlds there, including Earth, are interstellar colonies planted by Hain long ago.
So there are plenty of opportunities for the author to change things so that eating alien fruit would be normal for humans. After all, this is literature, not a chemistry or biology textbook.
But specifically in this case, the main character, Christie, was a human born on Earth. I don't think it was set in the very distant future, as cities like Karachi or London still existed on Earth in this book. There was also no mention in the book that humans had had time to change very much physically.
That's why I specifically quoted Golden Witchbreed and not any other book. It's one of my favourite books, by the way, and the fact that the main protagonist eats alien food doesn't make the book stupid. I was just trying to prove that sometimes sci-fi and fantasy are very close, and that books that are considered sci-fi can talk about things that are unlikely to ever happen in the future.
I don't think this only applies to very hard sci-fi. I think it applies to Golden Witchbreed too. Just because the science isn't spot on in every aspect - or in some cases is wildly off - doesn't mean it's not a projection of where the application of science might take us.
Of course, in many cases the science isn't spot on in every aspect, and that's fine. But a lot of sci-fi books are not about where the application of science might take us.
For example, many of Lee Brackett's or Ray Bradbury's books are set on Mars, where there are ancient cities and local populations. But obviously there is no science that could take us to such a beautiful Mars. Although I certainly wouldn't mind going there, because I like those books.