I know nothing of this method but below 4% or so, the myelination around the nerves is greatly reduced and really affects their fnction. Not to mention the hormonal disfunction that would be going on.
When boxing as yoof - young man I got below 5% and my skin was paper thin. To say BMI is inaccurate is unfair IMHO, as it for most people it’s a good guide. If you’re very muscular or endurance athlete (healthy) thin then sure it might say you’re at an unhealthy weight when you’re not. But if you’re in one of those groups then you’re unlikely to be unaware of that so why would you be interested in BMI?
Just used the calculator. It’s weird I put my values in and it gives me 16% fat but then I change the weight by 10lbs and the fat % remains the same. That’s moving it up or down. I suspect that like BMI, as the population of users grow the accuracy overall goes up (if that makes sense). This is the fattest I have ever been
The mirror values in this link are ball park right IMO and good enough for most I reckon. https://www.nerdfitness.com/blog/body-fat-percentage/