*Musk claims.
Not a reliable narrator, we'll have to see how this plays out.
If I had to wildly guess, Apple tends to be pretty strict about how an app moderates its content, and is not sure Twitter will adhere to its standards for what is allowed on their App Store.
I believe this is why Truth Social had a hard time getting on the App Store originally; they had to prove their intention to provide a baseline level of moderation before being allowed on (which they eventually did).