Except, as we have seen with Epic, and why we are talking about this case, is they can, by dropping your app from the App Store in retribution for offering an alternative. They can do that because there is no other app store on Apple and effectively prevents you from competing.
No, you are allowed to offer an alternative on your website. Netflix and plenty of other apps do this just fine.
If you want to advertise the alternative inside your app, well, that's a bit like Target putting a sign inside that mom-and-pop store saying you can get a better deal at Target.
Except, they did. Apple started the threats against Epic when Epic started lowering their prices for add-ons on other platforms, which made Apple look greedy. To bolster their anti-competitive case against Apple, Epic turned on the in-app store to which Apple then pulled Fortnite. Epic has to be able to show damages and anti-competitive actions in order to have a case. If they didn't force Apple to pull Fortnite, they wouldn't be able to show damages.