The type of illegibility matters, apparently. Decontrasting and visual noise stress a different part of the visual pathway to hard-to-read letter shapes. I'm trying to find the paper which describes this, but it's been at least a year since I saw it.

http://www.matthewfrench.net/pubs/fontspreprint.pdf reproduces the original effect. What's interesting here is that the effect is measured as significantly more pronounced in dyslexic students.