I just leaned over and asked Bruce, and either he misspoke or was misquoted; these things happen at conventions. There are some mechanics details in the books, and I have no doubt that you guys will find every single last one.

But their primary purpose is to serve as a primer to 4th Edition. The
Classes and Races book that I worked on, for example, starts with the timeline of the game's development and then presents multi-page spreads on most of the races and classes that you'll see in the
Player's Handbook. "Most," not all -- we want to save some surprises for later. ^_^ And as noted previously, those spreads are full of design and development commentary about the decision-making process for every aspect of the game.
Plus some amazingly kick-ass William O'Connor art. You ain't seen nothin' yet.