While I can’t say much about the specifics of Japanese health and nutrition, I’d argue it confirms the general tenet of dietetics that restrictive dieting is largely not good for you (and isn’t easily maintained either).

Eating too little (or unbalanced) taxes your metabolism to free up glucose from your organ stores and store what it has, plus running the risk of nutrition deficiencies too. Plus eating too much also has it’s obvious risks.

I think in regards to keto, the risks of high fat diets are independent from the effects of ketosis. You still run the risk of CAD, obesity, high cholesterol and the issues those bring. (It raises LDLs but lowers triglycerides according to a paper from the ACC, they and the AAND are not convinced one way or another it seems on if keto should be recommended)