“I am a participating writer in ‘Dune: Part Two,’” he revealed. “I came long after… You come in and do a little bit of work… It used to be that you couldn’t even say that, but now they have this additional ‘Literary material’ thing at the end, so I am ‘Additional Literary Material.'”
The “Additional Literary Material” designation is a fairly new WGA rule that allows folks like Mazin to come in and do some quick work on a script without mucking up the “screenplay by” credits. So, apparently, that’s exactly what he’s doing.