I must have missed something. Why would some other author's work be cannon for HxH? In most series, even other stuff done by the author themselves isn't (by default, at least) part of the cannon universe.
I think it's from this interview with Togashi and Ishida. In which there's nothing to suggest Togashi actually officially thinks of Ishida's work as an addition to his story. Sure, he says he got him right and that he planned to have Hisoka use the same technique he used in Ishida's one-shot. And he says he might some day draw Hisoka's story up to the point where he's lying on ther road before meeting circus troupe. But he also says not to expect too much.
Here's the interview.
http://chiggix.tumblr.com/post/145336753292/冨樫義博-石田スイ-特別対談-少年ジャンプ
–- Update From New Post Merge ---
I try translating the summary on 2ch.
! Chapter title is "The previous night."
! Many people gather near Black Whale Ship.
Nasubi Hui Guo Rou appears and makes a speech in front of them. Audience shout for joy at him.
Beyond Netero and princes also attend the party.
! Kurapika talks with Oito about the security of the ship. Mizaistom tells Kurapika on the phone that perhaps the meeting among Zodiacs before departure will be cancelled since they are too busy handling the chaotic situation (ex. many crimes) on the ship.
! Kurapika intends to find an evacuation route in case of emergency. He hopes Oito and her child will be able to creep into the crowd to escape from other princes and army.
! When Kurapika is about to get in touch with the system control room, incredibly dark aura leaks from the baby buggy although Oito doesn't notice it.
! Damn, not even babies can be just babies. I figured it might become more complicated than it seemed for Kurapika. It went way too smoothly for something not to happen.