That’s an oversimplification.
Although keto & low carbohydrates is an important topic, there are many other causes & solutions discussed in the book. For example:
1- Genetics & Epigenetics
2- Chemical Imbalances, Neurotransmitters & medications
3- Inflammation
4- Sleep, light & Circadian Rythms
5- Food, Fasting & your gut
6- Drugs & Alcohol
7- Physical activity
8- Love, Adversity & purpose in life
If I understand correctly, one of the main ideas is that mental disorders are metabolic disorders of the brain.
Then in part 3 of the book, Dr Palmer(author) explains the causes & solutions mentioned above.