Not coming in with proof but I have personally seen this- a buddy and I did some testing and the Ookla tests were the only ones we did that were much higher than the others.

I'm sorry for not contributing with hard evidence but I've seen this behavior (though I never heard about it from anyone else before now)

I used to work for a small ISP. The boss specifically wanted speed tests to be exempted from traffic shaping.

Not surprised. I have long noted that when my my browser is strugeling to load tabs all i have to do is try connecting to speedtest.net for all open tabs to suddenly load and i have decent speed for the next 15-30 minutes.