Aizen created the espada they’re not like Gin who was just a follower. The espada were his invention. Aizen knowing about the second release was hinted at when he was talking to Ichigo. And again neither stated Aizen didn’t know, just that Ulqiorra never showed Aizen. And even after Ulquiorra used it on ichigo, he still never showed it to Aizen but even you are saying Aizen witnessed it.
Not showing =/= not knowing. Ulquiorra is the number 4 espada. He himself acknowledged his place among them.
All in all it’s like Aizen said, all of Ichigos battle, even Ulquiorra took place in the palm of aizens hand.
