The LOTR "franchise" has lost all credibility thanks to Amazon. Bringing back the director of the good movies plus the world's best known Tolkien expert is a way to signal that the movie will stick to truth. That said Colbert doesn't know jack about screenwriting, it is like nothing else, not even writing novels (look at how JK Rowling did when she tried to write screenplays instead of novels) so I would question how much input he's really going to have. If I had to guess, it would be about as much input as George RR Martin has on his TV series, i.e., he attends a few meetings with writers and then fucks off and cashes his check.
I think Colbert's kid is a screenwriter and is going to be "helping," i.e. actually doing the writing, in this project. Colbert is most likely just gonna be the nerd providing the backbone.
That said, I don't know if screenwriting matters anymore. These things have to pass through so many different committees and meet so many different agendas before they get to the shooting stage that the writer has about as much say over the final product as the metal miners have over where the final bolts get set in a vehicle.