With that being said, CM did not promise (to my knowledge) to delete old private keys. But also, I think it is unlikely that CM had 6 TB worth of private keys stored on their servers.
There would have to be around 200 billion private keys on file in order for it to take up 6TB, which is an order of magnitude larger than the number of active addresses.
I suspect that it's mostly other data, eg: Running a full node will take half a terabyte, and metadata related to the chips; I wouldn't be surprised if that was a few TB as well.
I think the data is either some kind of logs or multiple copies of the bitcoin blockchain. The former would be used to trace coin associated with actual crimes and the later would be included in the complaint to scare others into potentially cooperating with law enforcement