FWIW, the article does include some data on the distribution, as well as address your exact objection:
> Since posting this experiment, I've received a lot of side comments along the lines of, "Well, of course I don't expect Bionic Reading to work for most people, but for [my subpopulation], it really works." If that were the case, we might expect to see disproportionate benefits for those participants who read faster with Bionic Reading than for those who read faster without Bionic Reading. Let's look at how many participants read faster with each font and their average speed gains.
Table 3: Summary speed differences per faster font
Count Percent Delta (WPM)
Bionic 998 52% 35
Non-Bionic 918 48% 43
> The number of people who read faster with Bionic Reading was slightly greater (52%) than the number of people who read faster without Bionic Reading (48%). That said, those who read faster with Bionic Reading only picked up 35 words per minute on average. In contrast, those who read faster without Bionic Reading picked up 43 words per minute. It does not appear that when Bionic Reading works, it really works.