Aizen can make himself appear to be anyone..he can manipulate the very world around him..To the SS Captains he could of easily made them think they were within the Soul Kingdom. Could of pretended to be a Zero Squad member and just waited for Yama to open the gate/portal if you will. I am not trying or attempting to troll here for Aizen's sake.
I could easily be misconstrued. but from the many sites I have read this from they all seem to translate he MADE Tousen his slave/subordinate....from the start..if it didn't mean that why would he place those words following his discussion with Unohara?