There's a very big difference between Kirby and an employee of Bell Labs. Kirby was basically an independent contractor, not a salaried employee.
Let's also not get too carried away with the idea that Marvel was a sophisticated company back in 1960. They basically were on the verge of shutting their doors. Standard practice used to be a statement on the back of a check, a practice that's since been ruled non-binding. You can bet that nowadays the contracts signed for "work for hire" are much lengthier and well thought out than in those days.
Again, it's pointless to declare victory either way since none of us is likely to have read the contracts of the day. That said, I wouldn't assume that a contract by a little company on the verge of bankruptcy in 1960 takes into account every change in the law to come.
Really, this is contract law. And everything in contract law depends on what's in the actual contract. If there is one. But, frankly, I expect 1. the Judge to rule whether the Kirbys have the right to reclaim and exactly what they reclaim 2. they move to settlement talks, if necessary, and as encouraged by the Judge. And this is all years out.