Sounds like confounding bias. Seven-Day Adventists abstain from cigarettes, a main contributor to early death. Also any diet that avoids the standard American diet will probably increase your lifespan, even veganism, though probably less so now with all of the processed vegan foods. That has no bearing on whether veganism is an optimal diet for human longevity, it just implies that the standard American diet is a poor diet. If you look at countries with the highest meat consumption - Hong Kong and Australia - they also have the longest lifespan.

https://pubmed.ncbi.nlm.nih.gov/6654568/ “It is concluded that evidence was found for the thesis that abstinence from cigarette smoking is the main factor explaining the low mortality from ischaemic heart diseases among SDAs”

I find it hard to believe there's enough of a population of Seventh-Day Adventists to actually say they have lower rates of prion disease.

Prion disease incidence is 1 to 2 cases per million people per year. You'd have to have millions of 7th day adventists submitting to research to be able to assert that with any confidence.

Vegan is more than avoiding meat. As far as health is concerned, I would put avoiding salmon, eggs, and fermented milk on the "makes your diet less healthy" list. I would be very curious to see a study that said avoiding once a week liver was healthier than having it.

Daily consumption of red meat? Yeah, the jury appears to be tilted against such behavior/habit.

What is the standard rate of prion disease?

You can't claim we know the knock on effects outside of calorie restriction.

If anything calorie restriction seems to dominate all other variables in all contexts but people don't like that answer.