I think there are two things: the short-term effects of missing a night of sleep persist for several days even if you get plenty of sleep later and the chronic

I think there are two things: the short-term effects of missing a night of sleep persist for several days even if you get plenty of sleep later and the chronic effects of missing a day of sleep accumulate even if you "make up" the hours later on. I'm guessing the second one is what the article is trying to say here. If you don't get enough sleep during the week, you can't make it up on the weekend, even if the total hours slept per week adds up. You'll still suffer the long-term effects of chronic sleep deprivation.