My father does not remember that locals got some particular health problems from that. Old people looked healthy and worked pretty much until the dearth. What is especially interesting that it was also true for descendants of Russian emigrants there from 100 years ago, but they did grow some vegetables and cereals.
Of cause, that does not prove that meat is healthy. But it does show that in a particular climate eating only meat is not harmful when the meat is from animals that was treated by locals with extreme respect and that were fed on grass that was either fresh or covered with snow at winter.
All bad things related to meat consumption are result of intensive and unhealthy farming practices, and combining increased fat intake with increased carb intake (especially fructose). Correlation, not causation.
That being said, there are two issues with meat: in the West, we pretty much only consider muscle as meat, and dislike the internal organs, when it should be the other way around. Even obligate carnivores prefer internal organs because they are richer with fat, and often leave a lot of muscle meat on carcasses.
Second, and related, issue is that muscle meat based diet is a high-protein diet, not high-fat diet, so it's more akin to Atkins than to keto diet. High protein diet comes with its own set of issues: it places significantly higher load on liver, which causes increase in liver size; obligate carnivores have extremely large livers compared to their body size (just google for an image of shark's liver compared to its body, it's almost 1/3 of the volume). This also applies to humans, for example Inuit have enlarged livers as well.
This is the result of increased need for protein-processing capacity. Liver can only process so much protein into fuel per unit of its volume per unit of time. And to use Inuit as example again, because of this, they keep snacking every couple of hours, more often when exerting themselves physically.
A related concept is 'rabbit starvation': eating lean meat (rabbit in winter) is not enough to keep a human being alive - while you're in theory ingesting enough calories to survive, your liver's capacity to process pure protein into fuel is less than your body's need for fuel.
The summary doesn't mention 'grass-fed', only 'wagyu'; wagyu is intensively grain-fed in the last year of their life or so.
To quote from a quickly-googled article:
"What many people overlook is that farmers make Wagyu as fatty as possible by feeding their cows huge amounts of grain for the last 300-500 days of their lives. Some farmers even add wine and beer to further increase fat content. The result is that a wagyu cow’s muscle tissue is thoroughly marbled with fat. Unfortunately, it’s the kind of fat that is not good for you. The mold toxins in all that grain are bad for the cow and end up in its fat, and then in you, which would mean wagyu beef has a disproportionately high toxin load."
Much better than the usual crowd I get in these kind of discussions, the 'sensitive vegan' type who pushes false statistics to try to hide being emotionally invested behind the authority of science (two of their favorite falsehoods are to push all people who consume meat into the same group for comparison vs vegans - bad science, because majority of omnivores have very bad diets incomparable to my efforts at keeping on keto, and arguing that 'cows emit methane, so we need to eat less meat', never 'cows emit methane, so let's eat other meat more'). And then when I display patience and deconstruct their arguments, the mask of reason falls off and they just start calling me Hitler, murderer etc. lol.
I refused, did some research on my own, and went on keto. My thyroid bloodwork is back to normal and I have no symptoms. And I feel much 'healthier' and more 'energetic' than before all this as well.
I am not proposing keto diet as a magical cure-for-all, but as a possible venue that should be tried before being forced to undergo irreversible surgical procedures for example.
P.S. body does not 'switch' the energy source to fat, it increases the production of ketones as result of interrupted Krebs cycle ( https://en.wikipedia.org/wiki/Citric_acid_cycle ) because oxaloacetate gets rerouted for use in gluconeogenesis (creating glucose for brain's needs), because there's not enough glucose in diet. This interruption causes the buildup of the main source for Kreb's cycle, acetyl-COA, which is then converted to acetoacetate (a ketone), which is then also partially converted to acetone and beta-hydroxybutyrate (also ketones). Of these, body uses acetoacetate and beta-hydroxybutyrate as fuel. Body is always able to use them, it's just that there's not enough of them to use if you're not on carb-restricted diet, so body defaults to using glucose as fuel.
Like you say, long-term effects of keto are poorly studied, mostly because of two reasons: early keto hype as an effective seizure treatment died out once we got good enough chemical seizure treatments, and there was a lot of money poured into blaming fat intake for negative health effects of increased carb intake from the 50ies (starting with Ancel Keys), and picking up significantly in 70ies.
There are animal studies that demonstrate increased longevity of animals on keto diet, similar to calorie restriction and strict veganism. While the calorie restriction and strict veganism most likely work through lowered intake of methionine (an essential aminoacid which functions as START coding block for protein synthesis, meaning that with lowered methionine intake your body creates protein at lowered rate and renews itself more slowly), keto achieves that through effectively tricking the body to go into 'starvation mode' i.e. the mode of using own fat reserves while the actual fat source is the diet.