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).