Actually I'm the one who put it there (to be exact, I sent the files to the teacher). You can find the syllabus of the class
here (French only), but in a few word the goal of the class is to teach first year, Life Sciences students what programming is through a biology-related simulation project that uses SFML for rendering.
I've never noticed the that it states "official" before actually... but the reason is extremely simple (as GraphicsWhale mentioned):
that's how the default documentation looks like. The fact that it look strange though is because it has the search feature enable.
I've been TA-ing this class for ~5 years now and can guarantee that at no point we intended to replace sfml-dev.org with our class website.
We have this online documentation because we need something that always exactly match the version installed on EPFL's virtual machines and avoid any confusion for the students.
Now, the current text also link to the official website but I agree that the wording is not perfect. I'm pretty sure the rest of the team would be happy if a pull request to improve the main page were to be made. :-)
@Hapax: I'm curious about the youtube video you've watched. Could you give me a link? Thanks.