"workers who are required to stand without opportunities to sit" sounds like it's little to do with posture and everything to do with the kind of jobs that don't let people sit typically being lower paid gigs for a different demographic...
It's also possible that these jobs tend to be shift work that completely mess up your sleep cycle. Shift workers have ~10 years lower life expectancy.
I was thinking about posture as well, including building bad habits, possibly affecting breathing too which would turn everything around on its head, raising blood pressure and inflammation, affecting sleep and then causing avalanche cascading side effects throughout the human body.
As they say, any health study that doesn’t control for economic status is just studying the effect of economic status.