Also once in solution, a fluoride ion is a fluoride ion. That means once in the water supply the fluoride from hexafluorosilicic acid is indistinguishable from the fluoride in sodium fluoride. The action from drinking fluoride and using say toothpaste is not the same. There is some overlap but they do not serve the same function. When you drink fluoridated water the fluoride ions stay in your saliva and act in a completely different way than a brief exposure to toothpaste or the like.
Fluoride in drinking water provides remineralisation, fluoropatite and bacterial acid inhibition. Fluoridated toothpaste provides fluoropalite and bacterial biofilm removal.