I have gone through their announcement but when it comes to picture how can they trace all the accounts who stores the bitcoin. There are many places where people are storing the bitcoin and in what format they found their address. I hope it is the very difficult job the Stellar group may be.
They don't have to trace anything. They take a snapshot of the blockchain, so they know the balance of every non zero bitcoin address. This is public knowledge.
Anyone who wants to claim lumens simple has to prove they own the private key to an address. Probably by signing a specific message.
I suppose if one only holds at an exchange or does not know how to sign messages, they are out of luck.
Somehow, this has to happen through exchanges like poloniex and kraken. That part I'm not that clear about but I suppose I'll have to provide an exchange address for lumens to be credited. Or perhaps an exchange username?