There are 3 possible scenarios and best speculation as to why the message was sent can be different for each.

First: Wikileaks was not compromised in any way, just as their twitter claims. Why send a secret message at all then?

Second: Wikileaks was compromised completely including their bitcoin account. Why would the attackers who want to fake the appearance that wikileaks is still operational send out a secret message like this? Thus far they have been dismissing theories using their twitter not secret vanity address messages.

Third: Wikileaks was compromised except for their bitcoin account which is still in the control of someone from the original wikileaks team. What motive do they have for sending a message this way? Are they under gag order? Why risk broadcasting a message at all?