I’m sorry but that can’t possibly be true, there is no way this process results in knowing the percent moisture in the filament.
If you take the desiccant out of this equation the filament will reach an equilibrium with the air at a given temperature, but the air RH from this process isn’t the percentage of moisture in the filament either. It’s not even the percentage of moisture in the air.
There are scientific instruments that accurately measure the moisture content of plastics. However they tend to cost many thousands of dollars. Wood can absorb large amounts of water, which makes it easy to detect and moisture meters tend to be inexpensive. However plastics absorb very little water — only minuscule amounts. Thus, the instruments need to be extremely accurate at the low end of the scale. When I look at the material science literature I see that PLA and PETG are characterized as being hydrophobic. That they don’t absorb water but rather that there are airgaps in the material where moisture can seep into.
When I have seen the graphs using these highly sensitive instruments, PLA soaked in water will immediately begin giving up its water when moved to a dryer environment and will tend towards equilibrium with the surrounding air. That is why those cheap hygrometers work, because the moisture in PLA tends to be in equilibrium with the surrounding air in a sealed container. Likewise with desiccant as it removes moisture from the air which PLA will then tend towards equilibrium. The air filtration in my house tends to keep the relative humidity at around 36%. I have never had a problem with wet filament even though I have left spools sitting on the AMS lite of my A1 for months at a time.
Polyamide is a whole different beast.