The question is really a matter of whether or not the author truly knew nothing of Battle Royale. As to whether or not it is possible, I don't know. You'd think BR, at the very least, makes its way around authors circles. I basically agree with MF though, in regards to Japanophiles. It's not always the case though; in regards to the Matrix, the Wachowski's have been very honest about Ghost in the Shell's [the film's] influence.
It never occurred to me to link The Matrix with Ghost in the Shell, specifically. However The Matrix is something that gets brought up time and again in undergraduate philosophy classes, as it's rife with a lot of easy to cite examples. I know the Wachowski's apparently said it was their "commentary on the Hegelian dialectic," though it's actually much closer to a Lacanian reading of Hegel with a heavy helping of post-modernism via Baudrillard thrown into the mix.
If they ripped off any particular source however, it's Grant Morrison's "The Invisibles."