A very interesting article, but it is disappointingly vague about what these UPS self tests are, and exactly how they cause disruption to the power grid. Figure 5b shows that their building's UPS has 20-amp spikes exactly every minute, which is a pretty big spike. My guess is that a UPS performs a self test by switching from line power to battery power. If most of the UPS units in a country do that exactly every minute, synchronized by NNTP [edit: NTP], yeah I can see that it would mess up the grid having all that load dumped at once. (Although the observed frequency disturbance of 1.5 mHz is extremely small, 3/1000 of a percent.) Now that the problem is identified, it should be trivial for UPS manufacturers to eliminate it.

(By the way, the article is in Nature Energy, not Nature which the URL suggests.)