The cross-referencing was very common in the old EU. Some of it was even requested by Lucasfilm. And some of it were just retcons. The Thrawn trilogy didn’t have those references, simply because it was one of the first things written.
Actually, if I remember correctly, Dark Empire was originally supposed to deal with the Alliance capturing Coruscant after the defeat of the Empire shortly after the events of ROTJ, but Zahn refused to reference the events that were supposed to happen in Dark Empire in his then incomplete trilogy, so Dark Empire’s events were instead moved to after the Thrawn trilogy with the Empire recapturing Coruscant for a short while (which was of course incredibly convoluted).
That’s interesting. I knew that Heir to the Empire and Dark Empire (first issue) came out roughly the same time, but I didn’t know that Veitch was forced to conform to Zahn’s ideas.
It did occur to me why Heir to the Empire (and the Thrawn trilogy) in general didn’t have EU references. As you said, there wasn’t really anything to reference at that point. But the main reason that Hand of Thrawn surprised me was that out of the books and comics that I have read it was somewhat more exaggerated and shoehorned in than the others. Even Dark Empire #1 which came out only a few months after Heir to the Empire had EU references (Thrwn, Han & Leia’s twins), but it was handled much better than what Zahn did later.
Basically what I’m saying is that the EU cross-references that I’ve read previously felt more-or-less natural to the plot, and sometimes they even went unnoticed by me until I actually looked up the characters/places. In Hand of Thrawn they felt random and sometimes even unrelated.