I suppose your right, there is nothing that says that the game is about the book, Although can't help thinking that it is based on the book, and even though came 10 years later was influenced by Tarkovsky's film version.
Solaris is a planet in the game, there is no storyline to the game. You control a spaceship and the main goal is getting this spaceship to the planet Solaris. I can't help but think this gives the image of simplicity to the book.