Could it be that this is where the coins ended up, in hackers new address or is it just the address of someone who used the same mixer?
They used the same mixer. Klee's BTC entered the mixer, the thief got some other BTC, and stolen coins were left in the mixer. To be exact, there are no "coins" in the blockchain, just inputs and outputs.
The stolen coins will now appear in other people's wallets for a long time. The system is designed that way, following one of the core principles of Bitcoin:
Fungibility. It doesn't matter if you have one coins or the other, they all have the same value.