Fact is that private keys which were supposed to be deleted were seized.
I don't think that was written anywhere, and I neither think private keys should be deleted at all. You should never delete a private key.
From their FAQ:
How long do you keep logs?
Your session lasts for 7 days. After that, your session and all its data will be removed. You can also destroy your session before time is up. We keep statistical data ie. how much was donated.
The fact that they destroy the session doesn't mean they destroy private keys. I'm confident that they must have deleted the logs, as they said.