That's only if you look at names like "Manu Ginobili", "David Robinson", and "Tony Parker", and judge them like the players they later became or previously were.
That season, none of those guys were anything all that special. Certainly, nobody on that team was playing at an all-star level outside of Duncan.
Is that really better than any supporting cast KG has had? Better than Cassell (19.8 points) and Sprewell (16.8 points), along with Wally Z? Or Wally, Chauncey Billups, and Terrell Brandon?
I don't think that that 2003 Spurs team is "considerably better" than what KG had at his disposal. In fact, I think it's probably worse than KG's best teams.
It's an interesting debate, and one I've been in a bunch of times. A couple things I'd point out to take into effect:
*Tim Duncan absolutely carried that Spurs squad in a way rarely matched. We only have +/- data going back about a decade, but over that stretch I'm pretty sure that no one player has had a higher playoff on/off-court +/- on a team that won a championship than the roughly +24 that Duncan had on those Spurs. He was dominant, and huge impact.
*That said, that '03 Spurs cast is under-rated by the fact that so many equate individual offense with quality play. With David Robinson (even diminished) for roughly half of the game, Bruce Bowen, Stephen Jackson, and Manu Ginobili the Spurs featured some of the best defensive players per position in the NBA even outside of Duncan, who was arguably the best defensive player in the league himself. Put that in Pops' system, and you had a defensive juggernaut of a team out there.
Similarly, that Spurs team went about 8 deep with legitimate offensive contributors. None great, but with their depth there was almost always a 2nd and 3rd player contributing solid scoring around the given Duncan. Now, it might be a different 2nd and 3rd player every game, but usually there was someone there.
The '03 Spurs were filled with players that were either past their primes or not quite there yet, but they WERE talented players that were excellent defensively and just decent enough offensively to support their transcendant superstar. Duncan did a lot with that squad, but they did give him something to work with.
*Re: KG's casts, I'd say that the '04 squad had a comparable cast to the '03 Spurs...when they were healthy. But with Sam Cassell (and Troy Hudson) both down with injury by the WCF, and Szczerbiak playing with a fractured back, that Wolves talent level fell way below the '03 Spurs by the time they were put out. If you want to compare KG's results with the '04 Cast to Duncana's up through the conference finals, I'd say KG was at least matching him. After that, all bets were off.
And the Wolves never really had Wally, Brandon and a contributing Billups at the same time. Brandon's career-ending injury early in the year was precisely what let Billups start getting real PT, and though he used that to prove himself he was still very rough as a point guard. Couple that with the general lack of talent at the other positions and the lack of a championship-caliber coaching framework (especially on defense), and that Wolves team was considerably weaker than either the '04 Wolves or the '03 Spurs.