I understand your skepticism about associational studies. Clearly, the likelihood ratio from seeing a positive result in such a study should be tiny in most cases. But just out of curiosity, if you automatically discount all cohort studies, where do you expect evidence on the causal effects of lifestyle interventions to come from?

Nobody questions that doing a randomized controlled trial would provide much stronger evidence, but a RCT with a lifestyle intervention as the exposure and mortality as the outcome would take decades to complete, would require a very large sample size, and would have several potential threats to its validity, including low adherence to treatment assignment and loss to followup. Furthermore, you would need a separate arm for every possible variation of the intervention, and you would need to do one of these trials for every possible lifestyle intervention

In the absence of an RCT, the best we can do is a properly designed and properly analyzed cohort study.

As far as I know, instrumental variables are the only other option that is seriously considered, but there are very few perfect instruments, and in most realistic epidemiologic settings, using a weak instrument is probably worse than doing a cohort study. If you want to go into a further discussion on this, as a starting point, see the article "Instruments for Causal Inference: An Epidemiologist's Dream?" by Miguel Hernan and Jamie Robins, and focus on the section on how minor violations of unverifiable assumptions can blow up the bias.

I am not suggesting that cohort studies are the answer, but rather that we only have four options:
Either (1) Conduct a lot of very expensive randomized controlled trials on every possible lifestyle intervention and wait a couple of decades for the results, or (2) do associational studies, or (3) Postulate that we understand physiology and biochemistry well enough that we can learn about the effects of lifestyle intervention simply by reasoning, or (4) accept that we are unable to learn about the effects on lifestyle interventions on longevity

Personally, I am leaning towards option 4, but I am willing to accept properly conducted cohort studies as weak evidence, at least to give us some idea about what randomized trials would be most promising.

What really confuses me about your comment, is that you doubt the rest of his post simply because he cited a cohort study, when it was obvious from just reading the title of the post that the only evidence he could possibly have on the effect of lifestyle interventions, would necessarily come from associational studies.