While I'm not privy to the specific contracts in question, who holds the copyright probably isn't relevant to the dispute, at least not in the most obvious way

While I'm not privy to the specific contracts in question, who holds the copyright probably isn't relevant to the dispute, at least not in the most obvious way -- namely, it doesn't matter that the copyright was never held by Foster. The relevant questions are, I suspect, (a) whether or not the contract included royalties to be paid to Foster, and (b) whether the obligation to pay said royalties survives transfer of ownership of the copyright. A lot of "work for hire" contracts don't pay royalties, just a flat fee, but I don't think we'd be talking about this at all if this were the case here -- Foster was evidently being paid royalties until Disney took over Lucasfilm. So the dispute is over the second question. Disney is implicitly arguing that they get out of paying royalties to every single author who created works for companies that Disney subsequently bought, regardless of whether those contracts specified royalties that the predecessor corporations would have otherwise been paying.