Federer's prime on clay started later than his prime elsewhere IMO. I don't think he was in his prime on clay in 2004.
Regardless, though, one match really does not show that much. This is especially true when Federer won their other meeting on clay in 2002 when Federer was worse and Kuerten was better. It is clear from that fact that Federer was capable of beating Kuerten in 2004 and just played a bad match against a player he should have and could have beaten. These things happen to every player, though they happen less to Federer than any other player in tennis history.
He was no longer top 30 caliber and struggling to win matches consistently on tour.
Kuerten was 28th seed at that tournament, which is top 30. He DID end the year 40th, but you'd be an idiot to think that Kuerten wasn't top 30 in the world at the time ON CLAY. Kuerten was a clay court specialist, so he might have been 40th in the world overall, but he was still top 10-15 in the world on clay; I guarantee it.
that very French Open couldnt beat David Nalbandian
Nalbandian is a VERY good clay courter. He has twice made the semis of Roland Garros. Why do you act like it is lame to lose to him on clay?
That also proves he is even weaker on clay then even those players who had an outside shot of beating prime Kuerten on clay- Kafelnikov, Medvedev, Moya, Corretja, Rios, and many others. Heck I am not sure if he even better than Sampras on clay anymore.
You are just a troll. Besides Nadal, Federer has DOMINATED all the great clay court players of his era on clay: Ferrero, Gaudio, Coria, Moya, Gonzalez, Davydenko. Federer is 18-0 against those guys on clay. He is 3-1 against Nalbandian. 2-1 against Djokovic. 2-0 against Del Potro. 2-0 against Verdasco. 3-0 against Ferrer. 3-0 against Monfils.
The fact that he is 1-1 against Kuerten does not change the fact that the only player who has been able to consistently get the better of Federer on clay is the possible clay GOAT Nadal. Kafelnikov, Medvedev, Rios, and Corretja were NOT better on clay than Ferrero, Coria, Gaudio, and Djokovic (against whom Federer is a combined 9-1 on clay). You listed Moya, but Federer dominated Moya while Moya was playing as well as ever. So I really don't think Federer would have had a problem with the players you listed.
But would prime Kuerten have been better than Federer on clay? Probably. But what's your point? Kuerten is one of the top 5 clay courters ever; it's really no shame to be worse than him on clay.