Specifically, your argument rests on the the invalidity of "Among high-legibility fonts, a study found 35% difference in reading speeds between the best and the worst."
And the reasoning you give, is that because it's comparable to this metaphor you wrote, "An understandable explanation: imagine having 5 dice. You roll each die 4 times, then compare the highest sum to the lowest sum. Then you report that the highest die rolls 35% higher than the lowest. This is what the authors did, with each die being a font. But the experiment does not actually show evidence of any difference. If you rolled 500 dice, this method could claim that the highest dice are 200% higher than the lowest, even though all dice are still equal."
It seems to me then, that the metaphor doesn't match the method in the original article. A more accurate metaphor would be (and I've taken the liberty to revise your text where my change is in brackets, if you don't mind):
An understandable explanation: imagine having 5 dice. You roll each die 4 times, then compare the highest sum to the lowest sum. Then you report that the highest die rolls 35% higher than the lowest. [Then you repeat whole previous procedure 352 times (352 is the number of participants reported). On average over all participants, the highest die is 35% higher than the lowest with some modest variation. So you claim that there is a 35% variation between the highest die and the lowest die for individuals.]
Seems much more reasonable doesn't it?