Thanks for the quick answer!
I bought about 80-100 RoS boosters so far, and my friends bought a lot too, and this was the first time we found a
Thorongil in one, so I don't think, that it is really an uncommon, it is rather a "bonus" rare, like the foil rares in movie block boosters. Also, if I remember correctly, there is one rare foil in every 9 booster, and this rate was perfectly correct so far, but in this 26 there was only 2, so it is possible, that it is in the place of a rare foil some way.
Btw, are there RF or MW foil Thorongils in RoS boosters too?