The man who mastered everything, but Bankai. I mean achieving your bankai is forming the deepest connection with your zanpakutou which I don't think Aizen has done. That's why he faked his bankai with his broken ass shikai. I don't find it as unlikely.
Aizen fusing with his zanpakutou due to the hogyoku doesn't mean he formed a bond with KS. That's just him fusing with the sword, he never did any inner self reflection like we see from Ichigo, Renji, or Shuhei. He only started doing that when he was alone in the Muken, but then again this is just my opinion. We really don't know.