There was some symbiosis, but it was mostly Shaq.
If it were all Rondo, then every center we have should produce the same results.
That said, other center that Rondo has played with has come close to touching Shaq's .66 fg% that year.
On the other hand, Shaq's fg% was also higher than it ever had been. Part of this was because he took fewer difficult shots and mainly just dunked, but credit Rondo for getting the ball to him in those spots.
So yeah, it was both, but mostly Shaq.