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.
There is a chance that the mentioning of Data was a speculation from investigators and there is a good reason behind it. Think, if they say that chipmixer holds 7TB, it will cause panic in people and the society will massively say that we shouldn't trust bitcoin mixers because they hold our data. This can damage bitcoin mixing business and make it hard for mixers to gain trust and operate.
If no one gets into trouble, there is a chance that what I said is true but we don't know for sure right now.