They likely released the source code as an incentive for other hackers to join in and find vulnerabilities, ultimately provoking even more damage to the customers or to the exchange itself.
This is just the meaning I read of their reason for bringing their source code to public: they have done their own damage, and they want other hackers to take their own share of whatever they can grab from the exchange data. Some are afraid to make their code open source, not just because they have something to hide inside but because they are not fully sure that all their database is protected and free of bugs that can be exploited. This hack group just did big damage to this exchange, which will take them time to repair.