Maybe the errors in the article are more a statement of how complicated and improperly communicated Unicode was... and mostly still is! While I think I understand most of how UTF-8 works, I still have to read and re-read how codepoints and planes and encodings and decodings work together. It's a pretty complicated beast that could very easily be misunderstood when it was less popular than it is now.
It's still widely misunderstood today.