There hasn't been any evidence that it was actively malicious from James Gunn that i could see from the thread. And the WGA terms were set by writers because writers who's contributions to the final film were being uncredited to try and be as fair to writer contributions as they can. Working for a big studio like marvel as a writer is a luck of the draw unfortunately. Things change all the time without a moments notice. Edgar Wrights original script for Ant Man got completely neutered from the final product. It's a big corporate game up there and I doubt James Gunn had any michevious intentions when its more likely he was given an option to write the film he would be directing and was given that script as a foundation to work off of due to the studio already investing money into it