SKT T1... versus Incredible Miracle. SKT could pick just about anything they want there.
Besides, nobody has said Cassiopeia is weak overall. They complained that she no longer feels like Cass and suffers early against too many matchups. In the hands of pro teams, who are depending on her in the hands of a great Cass player to carry late, yes, she's going to do well pretty often. Those teams build around allowing Cass to shine. Sometimes they'll use her top for easy farm, sometimes they'll camp, sometimes they'll counterpick mid.
NONE of that changes the fact that she is still weak early when part of her identity was being a lane bully. It doesn't change the fact that she wants to machine gun her E, when her core identity was poison mage. It doesn't change the fact that her passive is good scaling for late game, but is otherwise a completely uninteresting pile of stats that serve no other purpose than gating to make up for the SIGNIFICANT nerfs she got to her base abilities.
If anything, I hope that the fact such a boring and uninteresting champ is seeing this much play in LCS will mean Riot will finally realize just how much Stashu screwed up. Even when she shines, she is incredibly boring to watch. Her pick and win rates in normal play have not improved.