It's not a good representation. People who play crappy games like Football Manager 2011 also install steam and that are people that just have some standard PC. So there are a lot of people using steam that are not real gamers.
Besides that Crysis 1 was a game with great graphics. Because of that a fan-base exist around it. So people that are interested in Crysis 2 are more likely to have a higher interest in graphics and so a system for higher graphics. I did see some poll on this forum asking what people had. Just under 75% had DX11. A few had DX 9 and the rest had DX 10 or hardware that supported a higher version then DX9 but worked with XP.
People who play Football manager and said games are gamers as well, and are also the kinds of people who would be interested in playing Crysis 2, therefore they are just as important .
And that poll was BS. most of them were either: Lying because they think that DX11 = their DX10/9 cards will magically become better, or didn't know the difference and think that DX11 is a software only update. Not to mention it had like what, 100 people vote in it? The steam survey has hundreds of thousands, and is therefore a far more accurate representation of the current gaming PC.