Its pure BS, as you figured. The 30.8 HAS to be 410mm in length, thats the only requirement per Giant. I wondered why forever until finally figuring it out. With the shim, Giant KNOWS you have at least the minimum overlap needed for strength at this critical junction, regardless of how little(or how much)post you leave below the top of the seattube. With a 30.8mm x 410mm, this insures the needed overlap since the rider will certainly not run out of overlap by running alot of post...like I do with a med frame. Been running a 30.8 x 410mm Thomson on 2 currently owned NRS rides forever. Even cut it down to 385mm a year ago for weight savings. Still use it to this day. No worries man.
Pic from last spring. Only pic I have with the med framed ride w/ the 30.8 post thats been uploaded to my host site. I run the shim and an alien carbon post now, and this post is used on my large framed NRS. Less overlap is needed, hence the cut. Could care a less about the warranty. I'm 5 11.5" tall so I can use either frame.
<img src="http://www.fototime.com/ftweb/bin/ft.dll/standard?pictid={4F2D49D2-7B7E-4A39-99B9-0EAC5D217E2E}">