Note the word "aquifers", not surface water. Both the Meridiani results and the Marsis results show deep burial of previous surfaces followed long after by exhumation. This is not a new finding but the new feature is the chemistry that follows. Bury ice to 1-2 km (no problem getting ice on the surface of a cold planet, source from mantle degassing) and global heat flow (especially true back in Noachian) can produe warm water, if not pressured steam, resulting in a geopressured aquifer which can lead to phyllosilicates. Release a geopressured aquifer and you get catastrophic flooding as previously been postulated for explaining some of the erosional features on Mars. At Meridiani, a similar burial process, labelled as diagenisis, results in significant changes in the acid-sulfate altered sediments leading to the sperules. No claim has been made yet for burial depth but over 1km is possible. So I'd say neither clays nor sulfates are indicative of standing water bodies.