The crux of the case is quite simply the work flow process: Kirby worked on assignment from an editor at Marvel (ie. Lee). Kirby was not self-generating pages and bringing them to Marvel on speculation, with the freedom to walk across the street and try to shop them at DC or another publishing house if there was no interest.

The latitude of the Marvel Method which gave Kirby greater control over pacing, plot and the ability to embellish the work in no way changed the dynamic that he was working on assignment from the Marvel editor.

Based on the evidence available thus far, I do not see how the Kirby Heirs overcome that hurdle.