That’s a fun paper, because they basically find no actual negative health effect, then just ignore that fact and continue as if they did.
First, the global focus on the obesity epidemic has largely overshadowed the persistence of underweight in some countries. Our results show the need to address the remaining underweight problem
Second, although adiposity has been consistently shown to be an independent risk factor for several NCDs in individual-level epidemiological studies, at the population level, the effect of rising BMI on the course of mortality reduction has so far been somewhat small in high-income countries
All the study shows is that as people get richer they can overcome being underweight, which is an incredibly good thing - it reduces health risks significantly. Then it goes back to the usual moral panic shit about how that’s actually a bad thing.
Essentially everything you ever heard about weight and fat is ideological, not scientific, in basis. Research consistently finds results that aren’t statistically significant, then just blathers about how that statistically significant result would exist if they could account for x y z. There’s no scientific basis to the claim.
The basics of it are that extremes on either end, of being underweight or very overweight, are health risks. The health risk of being “overweight” are massively overstated, because people like to judge and bully fat people.