There was no theft of any private keys. The miners were hit with a man-in-the-middle attack that redirected them to a malicious mining pool where they kept receiving new blocks to work on, but were not paid their share for that work. The technical analysis is here:
Thanks. The thieves stole hashing power via the Stratum protocol. It has nothing to do with the Bitcoin protocol nor were Bitcoins stolen. The article is misleading.