Correlation vs causation leaves a lot open, chronically sick with a number of illnesses people may sleep more. Their mortality risk is also higher. I'd wish we could see controlled studies that randomly assign people to different sleep durations. You also have to control for gene mutations that affect sleep requirements https://www.scientificamerican.com/article/genetic-mutation-...
It's certainly possible that the relationship is not causal and can be explained by correlations with depression, unemployment, sleep apnea, etc. However I'm not aware of any study that definitively proves this. It would be great to see a study similar to what you propose.
In general I'm fascinated how too much of a good thing leads to increased mortality and small amounts of a bad thing leading to increased longevity via hormesis and would love to see if the same extends to sleep.
Some examples of hormetic longevity influencers: caloric restriction, alcohol consumption, exercise. Even DNP a straight up poison has been shown to be increase lifespan in animals provided the dose is small.
