Are w3.org docs correct? I'm referring to this section: https://www.w3.org/TR/compositing-1/#simplealphacompositing

Considering the formula co = Cs x αs + Cb x αb x (1 - αs) the docs state then, that co is the 


  the premultiplied pixel value after compositing


I would argue that this is not the case, since in this article the resulting color is depicted with a capital C and in the case of premultiplied version, the lower case is used (c). I'm not really sure if this is the case, it's not too obvious from both docs and the article. So, is the result of  co = Cs x αs + Cb x αb x (1 - αs) really a premultiplied value?
Yes, the statement in the W3C doc is correct. You can tell because the background term includes premultiplication by the background alpha αb. If it were non-premultiplied, it wouldn't have an αb in it.