If Jadzia possesses all the memories of her previous hosts, she must certainly remember the memories that Curzon would have experienced during his zhian'tara ritual, which means she should have known about the existence of Joran before the episode "Equilibrium". Is there an in-universe explanation for this?
Curzon did not know about Joran due to the memory block that @ewanm89 mentions; additionally, he never met Joran during his zhian'tara. The reason for this is never specifically explained, but it stands to reason it's related to the fact that the zhian'tara requires the symbiote's memories to be transferred by a Trill Guardian. Guardians, as we know from "Equilibrium", work for the Symbiosis Commission and they can sense blocks put on the symbiote's memories. The Guardian in charge of Curzon's zhian'tara obviously did not break those blocks during the ritual (whether this is because they knew the Commission's reason for the blocks or didn't want to defy their superiors is unknown).
Joran was blocked from the Dax symbiote' memories, until the block broke down there would have been no knowledge of Joran for Curzon, even during his zhian'tara.