The reason Netflix is allowed to do it is because they are classified as a reader app

> if an app is a way to exclusively consume content purchased elsewhere, like songs or movies or podcasts, they don't have to have in-app purchases.

According to apple an exclusive email client is not classified as a reader app and so they are forced to offer in app purchases. The guidelines are super confusing and in my opinion pretty crap but it's not a case of favouritism or giving special exemptions to Netflix for example