> Literally, I could do this, and I suck at design.
Are you sure about that? I've seen plenty of imitation XP interfaces in my day, and there are virtually always elements that are jarringly wrong. While I won't claim that MitchIvin XP is a faithful reproduction of XP, in the sense that one could compile a long list of inconsistencies with Windows XP, the experience is pleasant enough.
I think their point is that the skills of making a website which looks like Windows XP is adjacent to the skills needed to be a good graphic designer.
Pretty much most days I am the person who is taking a design from a designer and reimplementing that in HTML/CSS. I couldn't tell you where to start when creating a design, but as far as taking something someone else has created and reimplementing it in code? I can do that all day long.
The visual guidelines PDF exists http://interface.free.fr/Archives/GUI_Xp.pdf and turning that into a web page is just a matter of creating some DOM elements with the right sizing, margins, padding, fonts, borders, etc.
Keep in mind that something you most days may seem conceptually simple to you, but may be overwhelming to others. That is true even if they work adjacent to you, such as creating the actual designs. Perhaps a better question is: how difficult would it be to teach a designer how to implement it.