What the... Is this some sort of joke I'm not getting?

"The participants who did ultimately enroll, agreed with the knowledge that the aircraft were stationary and on the ground."

It's gently reminding medical researchers not to forget about participation bias (https://en.wikipedia.org/wiki/Participation_bias).

Another favorite of mine along these lines is "Cigarette smoking: an underused tool in high-performance endurance training". (https://pmc.ncbi.nlm.nih.gov/articles/PMC3001541/) This one might actually be quite pertinent in this case, because the FDA's decision appears to rely heavily on exactly the kind of reasoning that this article satirizes.

It's a multilayered joke.

There's an old joke about the lack of randomized controlled trials for parachutes. The joke is deployed when people complain about the lack of formal studies for things whose benefit is obvious.

Then somebody went ahead and did it, just to be funny. But you can't actually do a randomized controlled trial on parachutes, so you get a third layer of joke, about studies that don't actually prove anything.

As a side note, I've seen it mentioned when discussing the common question of "why no parachutes on commercial flights?".

The question has relatively simple answers and it's sometimes used in risk management discussions to explain threat models.