If you did read the paper, you'd have noticed that people who slept more regularly in this research were also wealthier, and more physically active. They did say they adjusted for these variables, but who knows. Maybe they didn't adjust them enough, maybe there are more covariates they didn't consider.
I don't know if they can have controls for "specific activity that changes their sleep schedule".
In the past few years I went from having relaxed 11AM daily meetings to rigid 8AM meetings, and my sleep has suffered immensely. But nothing else in my life has changed, so it wouldn't show up in my socioeconomic data.
I guess it just seems extremely obvious to me that people who live a predictable-enough life to sleep and wake at the same time every day are less likely to face mortality than people with schedules so chaotic they sleep at different times regularly. It’s still good science to do, but without a solid cause or two, I don’t think anything helpful is actually being revealed by this study, at least from a layman’s perspective.