I also take issue with how the article mentions that sleep is not a balance sheet, but doesn't really lay out that this was explicitly tested.
They said the participants slept only five hours a night for weekdays. 8 hours is the generally recommended amount for a good nights sleep, so to truly bring balance to that schedule, they'd have a 15 hour sleep-debt to make up on the weekends, in addition to the full 8 hours they'd need to sleep those two weekend days on their own. I seriously doubt the participants were able to sleep 16 hours on both friday and saturday night to fully pay the sleep debt even at their level of sleep deprivation, so I don't think you can really say the "balance sheet" approach is wrong.
I'm not necessarily saying I support a balance sheet view of sleep, I just think these kinds of articles are not very careful with the kind of wording they use. Also maybe I'm just living under a rock, but I don't feel like its that common to only get 5 hours of sleep every single weekday unless you're like a single parent with a baby (in which case you probably wouldn't magically get 8 hours on the weekend), I'd be more interested to see what happens if this was done in smaller spurts, like two consecutive days with three or four hours of sleep to emulate some work or school deadline approaching, but then 8 or 9 the rest of the week after its complete. As it stands now, the title of the article comes off as click bait because its a pretty fear-mongering conclusion drawn from what I consider to be pretty extreme circumstances.