It can't. He's talking about the 4g63 block and those numbers won't include the extra weight for the AWD system, intercooler, turbo, piping, etc.
We really have no way to verify if thats the actual weight either, since we're trying to ballpark numbers off the top of our heads.
I would think the 4g63 would weigh more than that because of the iron block, but then again thats why the block can make so much power with such small displacement.
My original theory hinged on the fact that 2000 seemed to think that the LS1 was some huge, heavy engine when in fact the displacement is large (or was large, given the standards of the time) but the engine has always been revered for being so powerful and lightweight.