Does this mean that the Cosby hackers, who had access to Satoshi's account, could have read our PMs?
Yes. I mentioned this at the time.
Have Satoshi's powers been revoked after the Cosby incident? What about Sirius?
Both of them are no longer administrators, though they can decrypt forum backups using their PGP keys. Satoshi's forum account can't be accessed anymore -- I reset the password to a random one after the Cosby hack.