That's just bullshit someone has made up for sure, saying that Sawano ran out of ideas when he has made over 1000 original tracks for animes, movies, series dramas and video is nonesense. He let Yamamoto cook because he was already working in other animes OST such as Bubble, Gundam Hathaway, Solo-Leveling, EIGHTY-SIX/86, in his solo piano album and in V album + other works. He already had over 100 tracks for aot.