By having the private keys, law enforcement was able to seize all the Bitcoin controlled by those keys.
If ChipMixer would have deleted them after handing out a chip, they wouldn't have been able to do that.
We do not know what it the content and source of 7GB data mentioned before. I may imagine the situation where CM operator
deleted private keys and/or session data as it was promised, but if server has been hosted using serious datacenter, regular backups were done. In my opinion it is not impossible that some data comes from backups (restore points) made by hosting operator.