This is true. But Kleef (
http://en.wikipedia.org/wiki/Duchy_of_Cleves) has been part of the Netherlands at various occassions and Dutch remained the official administrative language deep into the 19th century. Its dialect is called Kleverlands (
http://nl.wikipedia.org/wiki/Kleverlands, no English link, sorry) and is still counted as a Dutch dialect, not a German one.
(Eventually Dutch was forbidden by the nazi's in 1936 and this is was how it was stamped out. As a native speaker of Dutch, I'm able to still see which towns have a Dutch heritage as opposed to German by their official names.)
As for Cologne ... yeah, that's where things become really, really fuzzy. But as that area was never Dutch in any official capacity as far as I know, and it could go either way, I'm not making the case for that. (Even though King Willem 1 was offered the Lower Rhine, which he declined, heh.)