What percentage of that 11mg/L is sodium fluoride or fluorosilicic acid? I couldn't find any reference to this, but maybe I didn't know where to look.

In any case I find it kind of strange these studies do not differentiate between the two. And once you measure it in the urine, it is very likely too late to determine the original composition, which is in fact what the study concludes also:

Our findings must be confirmed in other study populations, and additional research is needed to determine how the urine fluoride concentrations measured in our study population are related to fluoride exposures resulting from both intentional supplementation and environmental contamination