The suggestion of FBG < 5.0 mmol came from mainly the following sources.
1) my endocrinologist mentioned that in passing, When my fasting level was significantly reduced from over 10 mmol to < 6 mmol within 1 month of diagnosis. He wasn't impressed...he says I need to go lower...
2) Dr Bernstein had observed the seemingly magical 83mg/dl (4.6 mmol) in normal healthy people for years. more recent CGM studies are showing the same...and I observe this (80-85mg/dl) on my last 3 day fast. RD Dikeman (Typeonegrit) blog mention the target in passing...
https://myglu.org/articles/our-journey-with-the-low-carb-diet-and-the-manual-artificial-pancreas
3) Jenny Rhul -
What is normal blood sugar?
4) Chris Kresser -
Why your normal blood sugar isn't normal
So basically, like our body temperature, blood glucose may also have a preferred/optimal very tight set point ( 4.6 mmol?) and the body tries hard to maintain that.