IIRC, Zahn was none too fond of direction the EU went in after the release of the Thrawn Trilogy – he never liked Palpatine returning in clone bodies, how KJA & co. developed Mara’s character, etc. – so he felt compelled to fix those issues in the Hand of Thrawn books. That’s why so many of those references are there.
Oh yeah, and he’s friends with Stackpole. They even collaborated on a story or two together.