I too have a 21.5" iMac version 11.2. And I have been looking for Apple to provide a fix for the waking from sleep issues that have intermittently plagued this computer since I got it 3 months ago. I was told by a genus that it is a known issue and Apple should be providing a software fix sometime in the future.
Apparently this is not it, since I cannot run the update either.
Same thing here. I have an 11,3 with the 2 TB Hitachi HD, and there is no update available for me. I downloaded it manually from the site, and it says that it is not needed. Fine by me, but I agree that Apple should state whether or not you need a certain HD or not, and not issue a blanket statement that makes it seem like it's for ALL mid-2010 models.