Cover thickness not clear
The scientists describe the ice as "relatively thick" and the cover as "relatively thin" out of a total thickness of up to 800m. Perhaps the cover thickness is below the resolution of the Radar instrument?
MARSIS (on Mars Express) nominal resolution is ~50-100m and it can penetrate to ~5km
SHARAD on MRO is higher relolution at ~10-20m resolution and penetrates to ~500m
SO the best answer is "a few tens of metres, or at most a couple of hundred m"
Some more snippets:-
Scientists aren't exactly sure during which past ice age the glaciers may have formed, but by counting the number of impact craters in the overlying debris, they estimate them to be about 100 million years old, said study team member Jim Head of Brown University in Providence, R.I.
From http://www.livescience.com/space/081120-martian-glaciers.html
I'm sure there will be a lot of detailed work now to establish the cover thickness from the crater population statistics and morphology and by using the higher-resolution SHARAD instrument to check the cover (even though it probably cannot reach the base of the glaciers)