You must be crazy. Coria, weak? on clay?
First of all Coria is not that great. 2004 was the worst year of clay court tennis of the Open Era probably. Only in such a weak year of clay courters could Coria be the guy to beat on clay for the year. Think about it, Ferrero wrecked by illness and injury, Kuerten long done with hip problems, a past his prime Moya, Federer not strong on clay at all apart from Hamburg until 2005, Nadal a 17 year old baby on tour who didnt even play the French that year. This was the grand clay court field of 2004, LOL! Federer was 2-0 vs Coria even at Coria's peak. In 2004 Federer could lose to anyone on clay, he still went out early rounds most clay court events he played, and yet he still beat Coria once he reached him. Ferrero bullied Coria in the Monte Carlo final on clay in 2003 when Coria was already near his best, and before Ferrero's health problems ruined him. 18 year old Nadal went 2-0 vs Coria on clay in 2005 which was Coria's last year playing top tennis. Coria never conquered any major foes on clay. His biggest ever win at the French was a 33 year old Agassi, and at his peak he lost to Verkerk, Gaudio, and pre prime Davydenko at the French. Coria did well only when he didnt have to play anyone that good.
If you look at the finalists from 1990 to today:
1990- Gomez and Agassi, both better than Coria on clay
1991- Agassi and Courier, both better than Coria on clay
1992- Courier and Korda, Courier better than Coria on clay
1993- Courier and Bruguera- both better than Coria on clay
1994- Bruguera and Berasetegui, Bruguera better than Coria on clay
1995- Muster and Chang, Muster definitely and Chang maybe better than Coria on clay
1996- Kafelnikov and Stich, Kafelnikov probably better than Coria on clay
1997- Kuerten and Bruguera- both better than Coria on clay
1998- Moya and Corretja, both probably better than Coria on clay in their primes
1999- Agassi and Medvedev, Agassi in his prime better than Coria on clay
2000- Kuerten and Norman, Kuerten obviously better. Norman in 2000 was probably better than Coria ever was on clay too.
2001- Kuerten and Corretja, both better than Coria on clay
2002- Ferrero and Costa, both better than Coria on clay
2003- Ferrero and Verkerk, Ferrero obviously better than Coria on clay
2005- Nadal and Puerta, Nadal obviously better than Coria on clay
2006 to 2008- Nadal and Federer, both better than Coria on clay
2009- Federer and Soderling, Federer better than Coria on clay
So the only finalists of the French that were weaker than even Coria on clay were Korda in 92, Berasetegui in 94, Stich in 96, a past his prime Medvedev in 99, Verkerk in 2003, and Puerta in 2005. So actually most of even the runner ups are in fact better than Coria. If he had won the final that year as he was heavily favored to do vs Gaudio, even he would have been the weakest French Open Champion of the last 20 years other than maybe Kafelnikov. Gaudio just was even a much weaker one still.