The person who wrote that is absurdly wrong. There's nothing inside D3D11 that prevents you from using an OS earlier than 8 (or 8 for that matter?). What he appears to be referring to, is when building applications in Visual Studio 2011, there is no C runtime compatible with Windows XP presently, and it's (IIRC) still presently questionable if there will ever be. Applications written in that version will only run on Vista or higher.

If you're targeting D3D11 alone, that isn't even an issue, as Windows XP can't use D3D11, and if you did want to target XP with D3D9, nothing is stopping you from using an earlier version of Visual Studio.