Thiem is far away from being the second best clay-courter. Maybe he was it in 2018, but now he has reached one Masters SF. In Monte Carlo and Rome he was a non-factor. Considering his usual Slam performances apart from RG 2018 I think he won’t be much better there.He will have a good chance of lifting his second French Open title if Nadal loses before the final or if he meets and beats Thiem in the semi final.
OP, don't rule out Thiem either. He truly believes he is the second best clay court player today, especially at RG. When he plays well without making many errors, he's one of the hardest players to defeat on clay.
And for his belief: In Madrid he said in German TV before the Federer match that he would never see himself as a favourite against Roger, no matter where they Play.