Cenae was definitely one of our top elite Pkers. But regardless of that, when most of us Sitters/Aes Sedai are sitting in the Hall waiting for a ceremony or discussion to begin, we are not entirely attentive to the game. Usually we are sitting in there waiting for others to log on and gather, and then once the ceremonies begin our attention returns to the game.
In this case, it seems the Aes Sedai gathered in the Hall were semi-afk, which is when Cenae took her opportunity to seal the session, blind one of them and weave the other dead. I'm sure they had both died before they even fully came back to their screens