Nice find, but I wouldn't say that "nobody knew" about the glymphatic system draining wastes from the brain. It's been a known-thing for more than a decade. This study does have some new findings.

One: "...we detect direct connections between lymphatic fluid channels along the cranial nerves and vascular structures and the cervical lymph nodes." (pretty sure the drainage connection to the venous system was unknown)

Another: "... age-related cervical lymph node atrophy and thickening of lymphatics channels in both dorsal and ventral regions, findings which reflect the reduced lymphatic output of the aged brain." (a possibly causal effect for age related dementia)

There's been some work (in anesthetized rats) showing that the system flushes/drains most effectively in a lateral (on the side - as opposed to face down or face up) position. There's no mention in the study you link of them having done any positional comparisons, but it seems pretty clear that they're also using anesthetized subjects). I don't expect they'll ever manage to do this with actually-sleeping humans, as NMR machines are painfully loud to be in, even with both earplugs and earmuffs. (I've been scanned in a couple - long story).

CT by comparison is dead quiet (I've fallen asleep in one - different long story) but, without contrast media, you don't get the needed soft-tissue imaging - and NMR, as it 'sees' phosphorus (protein) sidesteps the media issue.