I think the point is that a Rondo-led Celtics team can't win without Pierce on the floor and playing well (at least against other than the lowest competition) but a Pierce-led team can win without Rondo on the floor.
So what those two statements add up to is that we need Paul Pierce to win for sure and that Rondo is certainly a player that makes us better (as compared to Dooling, Bradley, Moore) but he doesn't make nearly the difference that Paul Pierce does.
This sounds plausible, given that the C's were 0-3 to start the season without Pierce, and are 3-1 without Rondo, but I disagree and I think the circumstances favor my point of view.
The C's at the beginning of the season were historically bad defensively. It was only Rondo's ability to run a high level offense that kept us in games (and Ray had a big hand in this as well).
Since then they have slowly improved defensively until breaking out into outright defensive dominance in the past two games. Meanwhile Pierce is doing solid work keeping the offense treading water without Rondo (though not quite at the level we reached early with Ray and Rondo).
So unless you credit Pierce with righting the ship defensively (or alternately, blame Rondo for upsetting it in the first place), I think the win/loss disparity between the Rondo-only and Pierce-only Celtics has very little to do with how much better one is than the other.