Wikipedia is a content-heavy website first, webapp only second. If it's written correctly, then of course it's going to work in IE5 - maybe with some parts of the layout looking ugly, maybe with some margins being wrong, some media not being embedded, but generally should be usable.

Nowadays it's fine for a webdev to not test their pages on IE5, but when the page is done right, there is no reason for it not to work in IE5, lynx, w3m or Netscape Navigator. Really, it's just a webdeveloper's job done right.