Sure, maybe it is a tad hair-splitty, but is it? Clarke was hired to write an original screenplay; it wasn’t meant to be based on another story. And the book wasn’t even meant to exist, initially. My understanding is that it does exist only because Clarke found script writing clunky and unnatural.

Although — even if the movie was based on a book — Kubrick would have done his own thing, and he wouldn’t have been wrong to take those liberties. Why faithfully remake a book? I can read a book. Give me something new.