What they mean is that they will not chronicle the events of the book, Reach is a planet and if you have read the book you would recall that the spartans IIs landed on one section of the planet trying to secure and defend the generator powering the MAC cannons. These guys are probably on another section of the planet.
As these guys are mostly Spartan IIIs, their existence is not public knowledge, what with ONI and their secrecy.
Like I've said before, they don't have to regard/disregard what happened in the books to tell the story in this game.
Actually, they seem to take into account a lot of what did happen in the books in this game, there is the mention of ONI, spartan IIIs, human insurrection, etc. And this is something I didn't think they would do.
Needless to say, as someone who has read most of the books I couldn't be happier.