A lot of British intelligence during WW2 was gleamed not from the contents of the messages they intercepted, but rather from tracking who was where and communicating with whom.
And if you stop soldiers from using mobile phones on restricted ground, you are just going to have lots of tracks stopping abruptly at the gates and secure facilities identifiable by their lack of emissions.
Patterns.
There have been great examples of correctly identifying the crews of nuclear submarines by their predictable periods of time offline.
In any case, the attack here was to identify personnel based on known locations, not finding new locations in the first place. Big bases can't be hidden anyway, the best you can do is conceal what happens indoor in them so it seems silly to let foreign intelligence track personnel movement inside a base...