Non American asking: What does crank mean here?

You've had some good answers, but i will add that the internet led to a Cambrian explosion of cranks, because now anyone can put up a webpage. There is a catalogue, although i don't think it's maintained any more:

http://www.crank.net/contents.html

"a pejorative term used for a person who holds an unshakable belief that most of his or her contemporaries consider to be false."