Richard Osman talked about it on a recent episode of The Rest is Entertainment, around the premiere of the film.
Amblin Entertainment (founded by Spielberg) bought the rights even before the book was published, so after that it was out of his hands what happened to it -- he had no involvement with the screenplay etc.
Originally it was supposed to be, I think, a cinema release, then Amblin said it was going to go straight to a streamer as with the cast it got attached, it became more expensive than originally envisaged. I think Netflix let them have a week's cinema release as a favour.
I have to say RO carefully avoided saying anything about what he thought of the film, and kept stressing that it didn't really feel like anything to do with him, and that he was always nervous about a book coming out, but didn't turn a hair about the film, and was even able to read the reviews quite happily, whereas he never reads his book reviews. I intuited that while he was delighted with some of the cast (he's very enthusiastic about Pierce Brosnan), he wasn't the film's biggest fan.