Without knowing anything about the actual settlement, my guess is that it's closer to something that gives the Kirby estate royalties for his 1960s creations similar to those other creators got for their creations from the 1980s onwards. Still a nice chunk of change but far from the controlling interest they were suing for.
I also think, ironically, the many amicus briefs were part of why the Kirby Estate would settle for less rather than gamble on the Supreme Court (and also why Marvel didn't want to risk it). Not because they helped the Kirby Estate's case (they didn't) but because they introduced a third, nuclear, option that neither party could stomach.
The Kirby Estate's case revolved around the claim that Kirby's case was unique and that everyone else's work was WFH while his wasn't (which would mean that if the Estate got rights to Hulk, Avengers, X-men, FF etc. then everyone else's work on those properties would also be blocked from use by Marvel). It is a bull**** claim for many, many reasons, but that was their argument.
The amicus brief, however, made the claim that Kirby's case wasn't unique and that nobody's work on ANY Marvel (or DC et al.) books at the time met with the WFH requirements. If that argument was succesful, it would atomize the rights involved in the Marvel Universe and practically every writer and artist at Marvel before 1978 could put their hand out or block some part of Marvel's back catalog. Which would also have the side-effect of making the Kirby Estate only one (though one of the largest) of the interested parties in what amounts to a class action suit for royalties. The potential take for the Kirby Estate could have been cut down to a hundredth, and that's after a lengthy and costly arbitration process.
It would have benefited a lot of OTHER people, though.
I don't see this as a vindication of the legal arguments for the Kirby Estate's case as much as risk management for the far more dangerous arguments in the amicus briefs. In that context, if the issue is creators' rights and one believes the WFH standard was exploitative and wrong, this settlement represents a clear and palatable loss of what is probably the last opportunity the comics industry will have to address this before the Supreme Court.
Do I think the Supreme Court would have picked up the case or that the Kirby Estate would have won, either along the lines of their argument or the emicus brief arguments? No. But I think Disney and Marvel may have been smart not to risk confronting the issue of the instance and expense test. Kirby alone? No problem.