They bought a rare Dune storybook, created by an all-star team of artists (Jodorowsky, Moebius, HR Giger, ...). I think that storybook was used to pitch the concept art to Movie Producers and Studios.

To make this easier to understand, substitute "Dune book" with "Gutenberg Bible" or, even better, "historic, ultrarare, pre-medieval parchment of a Bible translation".

The new owners of that Bible probably do not think (and do not intend to promote the idea) that they now own the property rights for "the" Bible, or for all Bibles, or for any Bible - except for that particular instance of the Bible.

Rather they now own that parchment, with all its decorated pages, handwriting, scripts, gemstones, peculiar translations, and compilation of Gospels and Letters.

These content, drawings and illustrations inside that Bible are really unique, unless the monks decided to draw everything ( or even more often.)

This content could indeed be monetized, or shared with the world, for the greater good.