Just to let you know you don't really "unlock" that much in the way of gun customization, I think it was Paul Wedgwood who said that "We give gamers all the big stuff at the start and let them unlock little things as they progress" or something along those lines. So in oher words everyone has access to most of the gun parts at the start of the game.
It doesn't mean that you have access to most of the gun parts from the start of the game, and it also isn't referring strictly to gun attachments, but abilities as well.
What he is saying, is that the "important" things, (options that a good number of players tend to choose and/or options that are typically reserved for high level players) will be the first things players in Brink will unlock. It doesn't necessarily mean that most things are available from the start.