It is possible that bitfinex did have a new cold storage key for every user, but I find this to be very unlikely as it would be hard to maintain for cold storage.
Rather it is likely that they had one cold storage master key and each users cold storage keys were generated from that. They probably had a derivation path of m/0/i where i would be a unique id for each user. Then the internal and external keys would be derived as normal from that prefix path.
Honestly speaking. I don't even understand even a single thing you are saying in this topic. It is the fact that, im not in this line of job. And im still studying and do not have tge knowledge about bitcoin, I just kbow that Bittfinex was hacked lately and that's it.