There are very few proper double blind studies on the subject, but there is a mountain of anecdotal evidence of people curing all sorts of metabolic disorders by adopting a carnivore diet. Research must be pursued in this area, but there are already a lot of signs that this is the right way to eat.
Discussion
True. The only radomized studies are short term with a specific focus. Hence, these can't really conclude on all-cause mortality/longevity impact.
For this it would require to put similar people into a lab for a very long time/life-long.
This can be done and has been done with animals, which again, are not humans and hence results may not apply.