Nah I stick by my score, it was a damn good movie. I can't remember the last time I enjoyed a movie as much in a cinema.
I don't know about the book but the movie very carefully avoids being similar to Battle Royale. Here's how, in BR we were thrust into the games within ten minutes of the start of the movie while in THG they take their sweet time getting to the games, almost a hour and a half. The buildup to the games was just so fantastic, all this bottled up tension. That's something I felt was missing in BR. Though the "survival of the fittest" portion was better conveyed in BR hands down. The concept of the games is similar no doubt but by deliberately not stressing on that portion of the book, the movie manages to distance itself from comparisons with BR.
As for the author not knowing about BR, I think that's far-fetched. She should have just owned up to it.