Applaud you for doing the research, and I'll probably end up referencing it in my work at some point. Surely there are a number of holes to poke (as with anything) the one that stands out to me at the moment is you didn't control for the race of the recipient. I.e. if your overall recipient list was 50% Hispanic, even though you randomized who got what, would still expect Hispanic-sent open rate to be higher.