We here have no way of knowing what passed between them privately.
It would certainly not be the job of Alan Dean Foster's representatives to make any part of Disney's case for them.
But the original contract might be worded in such a way that both sides think they are in the right legally. (Just because something is in a contract it doesn't mean that a court will uphold it. Then there is the court of public opinion. A lot of people are very ready to believe the worst of Disney. If they are fans of ADF's books, so much more will they be willing to side with him. But if the original contract is somehow at fault, it would be harder to convince die-hard fans that Lucas's organization was to blame.*)
I'd just have to know a lot more than any of us do seem to know for certain right now before I'd have the least idea which side was in the right legally. These things are often tricky and none of here are experts, so far as I know, when it comes to literary contracts and the law.
Anyway, my main point is that some contracts do have such clauses, and that anyone signing a work-for-hire contract with a large publisher or any kind of contract with a small one (or, really, any sort of contract with any sort of publisher) should be aware that there are such traps for the unwary and to be very, very careful what they sign. We have a lot of aspiring writers here, and I bet most of them are unaware of half the things they should look out for.
(*Why would such clauses even exist, some may be asking. What would be in it for the original company? Because if, at some future date, they found it expedient to sell on their assets it will make it easier to find a buyer if the company that buys those assets is not held to all the same obligations.)