The story material came before the screenplay
Urobuchi said he did not initially expect to continue Rebellion. During the period when he had stepped away from anime work for Kamen Rider Gaim, Gekidan Inu Curry delivered notebooks filled with rough drawings, settings and situations accumulated over time. The volume convinced him that there was enough material for a feature. Ideas surrounding the new Nazuka Sokone setting and the identity and motives attached to Walpurgisnacht were already present, according to his account.
Urobuchi frames his role as dramatic organization
Rather than claim sole authorship of every concept, Urobuchi described himself as connecting the notebooks' separate elements and proposing how their 'bones' could form one screenplay. That distinction is especially useful for Madoka Magica, whose identity has always combined his plotting with Gekidan Inu Curry's otherworldly imagery, Akiyuki Shinbo's direction, Ume Aoki's characters and Shaft's animation. The interview therefore attributes the new film's narrative architecture more precisely without erasing the credited screenwriter's structural work.
Production kept changing the written plan
Urobuchi also said Shaft added the convent-era flashback during production, evidence that the finished film was not a fixed transcription of his draft. His testimony is authoritative about the material he received and the work he performed, but it cannot by itself catalogue every later contribution or settle all questions of authorship. The next useful evidence would be equivalent accounts from Gekidan Inu Curry, Shinbo and the animation team, especially on which ideas changed after scripting.