That makes no sense whatsoever given the many, many things in the Thrawn trilogy that are impossible given newer revelations that have come up in the prequels and the Clone Wars.
Like what? Name a single discrepancy from the Thrawn novels that hasn't been fit into the larger continuity with the prequels?
Spaarti Cylinders instead of Kaminoan technology?
An older, more dangerous cloning technology that was hauled out by the Republic when Kamino's cloning facilities were finally shut down/destroyed.
Pellaeon talking about fighting clones while in the Republic Navy?
Established that the Kaminoans had tried to create their own clones to rebel against Republic control, after the Republic had put a garrison on the planet during the war to protect it against the Separatists, but Kamino wanted to remain independent.
The timeline established for when Honoghr was poisoned?
This has been retconned that the years given in terms of how long ago were Honoghr-local years, not galactic standard years, so the poisoning could have happened 40+ local years ago, but only happened 27 years ago by normal reckoning, heck the poisoning of Honoghr was actually shown somewhere in the various Clone Wars media.
Those were the big three that I knew of, and authors in the EU went far out of their way to retcon things in. Every little discrepancy that the prequels introduced came along with fixes to blend it all together. Authors intentionally were vague about the clone wars era because they knew Lucas would one day come along and write stuff there, so they didn't put a lot there, and left lots of wiggle room where they did.