Otto

The paper you cite is from the long since discredited publication, "Fluoride", a magazine of the antifluoridationist group "fluorideresearch".  "Fluoride" is nothing more than an outlet for poor quality antifluoridationist literature.  It is not even indexed on PubMed.

The issue of incomplete hydrolysis of silicofluorides was put completely to rest by Finney, et al. In 2006.  Using NMR technology, Finney definitively demonstrated that there are no intermediates of SF hydrolysis down to a pH level of 3.5.  The pH of drinking water is 0.7 ppm.

Reexamination of Hexafluorosilicate Hydrolysis By F NMR and pH Measurement
William F. Finney, Erin Wilson, Andrew Callender, Michael D. Morris, and Larry W. Beck
Environmental Science and Technology/ Vol 40, No. 8, 2006

Steven D. Slott, DDS