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.
Day 1: I exchange bitcoins via Addy1. Attached to Addy1 is a b-code so that I don't get my nefariously-obtained BTC back.
24 hrs later: The transaction is purged from Bitmixer's servers.
Day 3: I exchange bitcoins via Addy2. Bitmixer gives me others' BTC with no chance of me getting coins from Addy1 thanks to the b-code attached to my account.
See the paradox? It's impossible for them to do my Day 3 transaction if my Day 1 transaction was purged from their platform as promised but fully capable of performing such due to some b-code. Ergo, if there's no record of the first transaction, then it's highly probable that one could obtain coins that they didn't want to be attached to. It can't be 100% possible to have it both ways.