The ruling didn't, and couldn't given the ICJ’s jurisdiction, order Hamas to do anything: https://news.ycombinator.com/item?id=39149823

> Hamas has previously claimed they would abide by any ruling of the court

No, Hamas previously claimed that they would observe a ceasefire if the court imposed one on Israel, conditioned on Israeli compliance with the same. They didn't say they would do anything related to anything other than an ceasefire order.

Hamas is not a state. ICJ is a tribunal that adjudicates disputes between states. ICJ has no jurisdiction to rule in regards to Hamas. Neither can Hamas bring a case against Israel, as it has no standing, considering it is not a party to the international treaty.

Palestine is considered occupied territory and is not a part of the UN

They probably claimed it because in their delusion they thought the court would unconditionally side with them.

Unlikely that they will release the hostages just because a court said so.