I firstly want to thank all readers who commented on my recent post about "The Passage" - a book I have not read, but apparently, the vast majority of readers have. The responses are varied and fascinating, and I feel this subject deserves further discussion.
Most of you don't think "The Passage" is sci-fi. A few examples of other "non sci-fi" comparison books were offered, books like "The Road" or "The Stand", books that are described as "post-apocalyptic" or "horror". While horror may fall into its own category, post-apocalyptic or dystopian novels do, in my mind, fall under the vast science fiction genre. This explains why I keep seeing people explicitly excluding science-fiction from the description of "The Passage" - we have different definitions of science fiction. This makes a lot more sense now.
Another somewhat recurring sentiment is that the book can be split into sections with different genres. This is really interesting to me, mostly because I never think to divide books like this. And, again, it further explains the gap between my mentality and the majority. By dividing the book into sections, readers see parts without any obvious science fiction as purely "literary" or horror, or romance, therefore explaining the fact that this book is many genres in one.
And a lot more fascinating thoughts you raised. Thanks to all who commented - though I'm not fully convinced that "The Passage" does not technically belong to the sci-fi genre, I do realize that everyone looks at things differently, and ultimately the genre shouldn't be defining the book quite as much as I assumed it to be. I do, however, get the impression, that "The Passage" is a good book. That should probably be enough.