"anyone could've created that username on slashdot"
Yes, it just happens that a guy named Carmack from id is the one who did

This is far from the first time that Carmack has posted on /., if you follow news closely you will see that every couple of months he tends to throw up a post over there about something or other.
Why would he say something like that? He is a geek, he likely cares a great deal more about video cards then the majority of people in this forum, he has a rather large financial stake involved in them. Doom3 is going to need NV20 class hardware to run properl so likely he is enjoying what it can do a great deal(getting to see what it is he has been working on).