In fact, he is actually the only player to defeat Rafa this year on clay.
He was the only one to do that in 2017 either. I wonder what else (apart from winning RG) the guy could do to make a statement. In 2017, not only he managed to beat Nadal on clay, he gave him a helluva fight in the Madrid final, probably the best clay match of 2017. My point back then was, and still is, he is the only "younger" guy who showed to be able to face a big three player
playing close to his best. (Maybe Zverev did that in patches in the Rome final, but I honestly think Nadal heavily contributed to that).
The historical odds between those two are 1:2. Given the RG final factor, correct it to 20% chance for Thiem, as the bookies say, and it really sounds reasonable. All points in GSM's post from which I took the quote are a very good reasoning for this number.
I see it this way: It is usual in Nadal's career, and specially at this particular tournament, to have a slow start. Not all players could benefit from that, and, specially, not a single one was able to make at least competitive the moment Nadal raised his level. This is precisely were I think Thiem can make a stand, to fight to protect an early lead. If he is able to do that, he has a shot.
So that's the first big if. I could be extremely optimistic and say that Thiem is the mild favorite to win the first set (say, 60% to 40%). The second big if, in fact much larger than the first, is how he would deal with the eventual lead.
Luckily for Thiem Nadal is playing bellow his 2017 level. Thiem is too, but I think the trade off is better for him. We already know Nadal is able to maintain or even increase his level on a major final. Thiem still needs to even try it...
I think that Thiem makes a statement, shows the world he is the next big player on clay, but sill loses on a hard fought 5 setter that will cement (if that was needed) Nadal's legacy and will be seen as a dividing point in tennis history by future tennis journalists, historians and, much more importanly, (this) board posters.