Clearly, my Bitmixer code remains on file attached to bitcoin wallet addresses I've previously used, else there would be a strong possibility of me receiving some of the coins back that I may not want attached to me, but that would be an impossibility given that ALL order data is purged from your system within 24 hours, as stated above, resulting in the 3-LAs et al. finding nothing.
I don't know if this is how they do this, but if the "bitmixer code" is a hash of an address, because hashes are one-way, that should be able to provide the basis for a system where they're not storing your old "from" addresses, but can check when you ask in the future that they aren't re-using anything.