It is not practical to do an RCT on long-term diseases.

We know the reverse causation hypothesis is not true:  that dying young causes people to eat meat earlier in their life.

So the only other hypothesis that I'm aware of is that some third factor causes people to both eat less meat and to live longer. That is called a confounder. And epidemiologists have nightmares about the confounders that they didn't think of.  There are always unknown unknowns, so we can always be wrong.

But proof isn't what we should be looking for because science doesn't give that anyways.  We should look at the weight of the evidence and decide what seems most likely.  My read of the evidence is that it is highly likely that high meat consumption causes shorter lives, and even probably because of the saturated fat and not from something else.  That is, I believe it is highly likely that coconut oil would do the same thing.

BTW: in studies on atherosclerosis in animal models, do you know what the standard method is to create a rat or mouse with atherosclerosis is so you can do an experiment on an atherosclerotic animal?  It is to feed them coconut oil.