This is patently false.

Martin Berkhan created a very lucrative coaching platform with Intermittent Fasting: https://leangains.com/

Many books have been written on IF for profit:

https://www.amazon.com/Complete-Guide-Fasting-Intermittent-A...

https://www.amazon.com/Eat-Stop-Brad-Pilon/dp/177511080X/ref...

And countless others.

I recently started a keto-style (high fat low carb) diet after reading a book on fasting [0]. I also picked up a ketone/glucose meter out of curiosity. Much as the book describes, fast for about 24-36+ hours and there's a good chance you'll be in ketosis.

After about 36 hours I've consistently measured 1.3-6 mmol/L. Between 48-60 hours I've measured around 4.4-4.6. As I understand it, anything above 0.5 is essentially ketosis. I agree with you in that, at least to me, there is a feeling of when you are in ketosis.

Ketosis is the generation of ketone bodies in the absence of glucose, ketoacidosis it the generation of ketone bodies despite the fact that the bloodstream is flush with glucose. Ketoacidosis is clearly a real concern, however as I understand it it's primarily an issue for type-1 diabetics.

This article feels awfully superficial, but I find the final statement, "maybe not being in ketosis isn’t so bad after all—now just cut back on the saturated fats", to be very reasonable.

[0] https://www.amazon.com/Complete-Guide-Fasting-Intermittent-A...