They pay 0% because they don't offer the ability to sign up in the app at all. If they did (Netflix and Spotify have in the past), they have to pay the 30%. To offset this, the price is more expensive inside the app. You can see this with YouTube Premium: £11.99/month via the web, £15.99/month via the iOS app.

All the "reader" designation does is allow this off-site option to exist at all, still with the restriction that you are not permitted to even imply that you can sign up elsewhere.

The only thing this does is make the customer experience worse. Now we have a Netflix app that can't tell you how to sign up, and a Kindle app that can't sell books. The alternative is the customer has to pay more, or the company has to eat the cost. Nobody wins except Apple, but you could make the argument that they're harming their own platform with this stuff.

To be fair, you can’t sign up for a Netflix account on iOS. Arguably the entire transaction is taking place off platform.