Walking has a direct casual impact on markers of cardiovascular health like vo2 max. I have no idea whether anyone has the funding to establish causality for exercise and all-cause mortality, but it would be astonishing if exercise improved your heart and circulation but not lifespan.
Edit: At least, astonishing if the effect persisted across multiple large and high quality studies.
I'm not so sure. I've seen some rat studies on longevity and fitness with pretty surprising results. Obese, idle rats were the shortest lived (not surprising). But surprisingly, fit, active rats actually had shorter life spans than anorexic, lazy rats. Who knows if it generalizes to humans, but results like this always make me skeptical that first principles thinking will hold up in complex biological systems.
I think this is also first principles thinking going wrong: calorie restriction increases rat longevity and (presumably) reduces rat movement, and there's no good way to make calorie-restricted rats voluntarily do more exercise without introducing confounding variables, but that doesn't show whether a calorie-restricted human would live longer if also doing some exercise.
There becomes a point for individuals where even walking becomes difficult, if you can imagine a sudden death is often a result of cardiac arrest and heart disease, then no it has nothing to do with walking but the issue of obesity. For most individuals in this category walking is considered strenuous exercise. Meaning that even doing 30 minutes of it can be considered the equivalent to a high intensity workout. It's a sad thought to think about about approximately 1 in 4 Americans are already on track for this by 2030.
Exercise is essential yet in our modern lives it does not exist in a mandatory form. The entire workplace structure of our society needs to be redone.
It is a interesting question. Some percentage of the harm from smoking is probably that it makes you more sedentary and likely to avoid cardiovascular activity. People who walk briskly for 22 minutes probably also eat better.
But how do we know this destined-to-die cohort will change that destiny by merely pretending to be the other group insofar as walking >=22 mins?