The Spurs are THAT good. However, they are also in a similar position to the C's in that they have a lot of old and injury prone players who they can't really afford to lose for long stretches of time.
I think the C's and Spurs are very evenly matched when fully healthy. The problem is, the chances of them both being fully healthy come playoff time is slim.
The key difference I see between the Spurs and the Celtics this year, though, is that the Spurs have been able to succeed even when neither of their older stars - Manu and Duncan - contributes much.
The Spurs have a lot of young, talented players who can contribute at a high level on a regular basis (Parker, Hill, Neal, Blair - plus Richard Jefferson isn't really old).
The Celtics don't really have the same luxury...while they have young players who can contribute if they are healthy (Rondo, West, Robinson, Davis, Perkins), they can't succeed unless they get a significant contribution from at least one of their older players (the Big 3 mostly).