They didn't use any JavaScript to get this information. Most of it is sent by your browser in the request headers, and the font detection used Flash and Java. They could have used JavaScript to detect fonts as a fall-back when Java and Flash are disabled, but it's relatively complicated to do so (requiring you to know the rendered width of a string for each font you're trying to detect), and it was not included in this example.

Sending incorrect information for Java or Flash fonts is an interesting idea, and likely would not affect user experience, as non-standard fonts are often served with the animations. Sending the wrong screen size might get you a mobile site served when you were wanting non-mobile or vice-versa. IP address and ISP are valuable bits of identifying information as well, and those are more difficult to address without using a proxy. But I would bet that randomizing your screen size for each request would break most fingerprinting code, since that would be assumed to be static.