I had thought the same, but measurements prove otherwise.
Edit: I should say, that measurements prove otherwise - but the position of the speakers was also slightly altered when the panels were installed. I should also note, that the panels affect the treble, not the bass, for which they are indeed much too thin.
Since I'm moving, I decided, that taking some measurements demonstrating the effects of the egg cartons was in order. The results in the first graph are as expected: the bass remains unchanged, whilst the mids have a non-trivial difference; the highs don't seem especially bothered. The second graph shows the results a bit more clearly. Note, that the left speaker was touched as I attempted to remove the egg cartons from behind it: I use this to explain the large spike at 17 Hz. The right speaker, so far as I know - and indeed, the rest of the room - was untouched.