We still don't know if the hackers didn't steal all the DMs to sell on the dark web. They basically can prove without a shadow of a doubt to anyone that they were the hackers involved (by sending BTC from the publicized address). That removes any questions of authenticity for potential DMs and is likely to increase the price. They or someone else they resell to can then go about the blackmail aspects of the whole thing.
If they didn't mean the BTC address to be an authenticity stamp after the fact it seems silly to not have varied it to get around blocks.
edit: They can also use it for blackmail even if there's no incriminating DMs. By making up fake DMs and then using the authenticity stamp to "prove" they were authentic. Could cause quiet a bit of chaos if released in the right way and be worth something to someone.
If they didn't mean the BTC address to be an authenticity stamp after the fact it seems silly to not have varied it to get around blocks.
edit: They can also use it for blackmail even if there's no incriminating DMs. By making up fake DMs and then using the authenticity stamp to "prove" they were authentic. Could cause quiet a bit of chaos if released in the right way and be worth something to someone.