C'thun the shattered didn't shattere

I was in a duels game and my opponent drew c’thun the shattered after the mulligan, but before his first turn draw, because of a treasure and c’thun just didn’t shatter.

I also have a link for the game in hs replays, but can’t post it, PM me for the link

thank you, here is the link for the game

https://hsreplay.net/replay/Np2tT2uDijYUpScibUKjFZ