One has to wonder how that affects the validity of sleep studies, or at least their applicability to the general population.

it would seem likely that if they discover a physical mechanism at work in select individuals brains (electric waves followed by rhythmic waves of fluid) that the mechanism exists in some form in all individuals. I agree it does make for a clear follow up question: "could this mechanism be altered in some ways in people who experience sleep problems?"

Or even "could this mechanism work differently in people who are unusually easy sleepers / deep sleepers vs. the general population?"

This was not true in my case. I cannot meditate easily if I hear conversations or erratic noises, but the steady rhythmic nature of the MRI machine integrated effortlessly with my meditation. The staff thought I was sleeping, and said they'd never seen someone sleep through such noise.

For me it is fairly straight forward to meditate even in noisy environments where I could not sleep. But I have no idea how prevalent that is. Certainly many people meditate in dark and quiet spaces (I worry I would fall asleep in such spaces but again, that's just me)

Yes, there are many noisy environments where it is easier for me to meditate than to sleep, and some in which the reverse is true.