That's not true. The same byte sequence is either valid UTF-8 or it isn't. One of UTF-8's distinguishing features is that you can tell with fairly high probability whether a sequence of bytes is valid UTF-8 or not. Unlike (say) ISO-8859-1, which could be ISO-8859-2. You can't (easily) distinguish without human intervention.
The fact that the site declares itself to be both UTF-8 and iso-8859-1 is just ludicrous however.