No, it's not about just having the idea for floating mountains, it's about being able to establish that Cameron himself publicly said he drew ideas for Pandora from Dean's Yes album art and that --according to Jan's
post-- members of the Avatar
production staff have said they were given copies of Dean's work to serve as inspiration for designing Pandora as well. That makes him like a de facto concept artist on the film, and thus deserves proper credit and compensation.
If there had simply been floating mountains, giant lopsided trees, and colorful dragons in the film and no public acknowledgement that Dean was a muse for the art staff on the film, then he wouldn't have much a of leg to stand on. But that's apparently not all that happened. If you can acknowledge that he inspired your work, you can give credit where credit is due on the final product. I admire Cameron as an artist, but he does seem to have a recurring issue where this is concerned.