Sounds like sour grapes and rationalization. The producer states that his complicated projects failed. If all of your complicated projects failed, then it may be that you struggle with making complicated projects, not that Americans don’t like complicated projects.
Plus, it sounds like he disproves his own point without realizing it. He simplified the Witcher and it still isn’t doing well. Isn’t that an indicator that maybe plot complexity isn’t as strong of a predictor of audience engagement as he thinks?