If Lucasfilm ever owned the copyright to these books, they could grant themselves the right to publish without obligations. They didn't, so they never owned the copyrights, so Disney doesn't either.

After all, a contract to let somebody publish your book in exchange for royalties is basically a license agreement, right? The copyright owners granted Lucasfilm a license to publish as long as certain obligations were met, same as when you publish code under GPL. Disney's position is analogous to saying they acquired the right to use your GPL code, but not any of the obligations the GPL imposes. That's why it's absurd.