Personally, I think the advice in that article comes with a risk. They've pretty much given you instruction on how to negatively position content off the screen and at the same time hide it when viewed with a text browser. Bad advice from my perspective. And something that could be part of an automated filter. If Googlebot parses a CSS file and detects negative coordinates placing content out of the viewing area, and in this case hidden, you can figure out the rest of what might happen.
Even though the content being hidden is relevant, there would be many more cases of where it was being used in a manner for which it was not intended. When it comes to automation, the majority rules. ;)
Hmmm, the tagline for the referenced site is "Web Accessibility in Mind". Someone forgot to take into consideration the potential risks involved in recommending this from an SEO perspective.
Also, after reading that white paper multiple times, I'm going to personally recommend that anyone following this topic not implement what they are recommending. This is a classic example of hiding content using CSS and absolutely positioned negative coordinates. Old school stuff that still works. :(