I am sure he can prove what in the film came from his book and what didn't. The article specifies much of the book was written from his own sourcing.
If there is even 1 thing in the film that cant be sourced to a 2nd published article, than that proves his book was in part adapted.
That is always possible, but in this case I suspect that isn't so.
This author has independent research and can likely prove that nobody before them ever presented the story in the structure of a spy thriller.
The author can also prove the Tetris company had the galleys before publishing. A professional studio would have likely never allowed one of their screenwriters to ever read the book. I think this case.is going to get settled.