If I may, it seems both sides are arguing different points. You seem focused on the low-level specifics of that particular case, versus the illustration of new laws being introduced retroactively as a high-level point on principles.
For mine, the crux of the argument is whether the three dominant platform holders in Windows, iOS and Android are too influential in global market economics to be tantamount as public utilities or not. They have unrivaled positions of power as gatekeepers. Without going down this particular rabbit hole, did we want Apple, Google, and Amazon to flex their collective muscle and say Parler as a business is not entitled to exist?
To not be allowed in the App store instantly deprives your product or service of access to over a billion users. I fail to see this as an arbitrary figure that is laughable.
But that is not true.
If you don't want or cannot sell on Apple, you can sell on Android. Even if Google doesn't want you on their play-store, there are other freely available app-stores and you can even implement your own copyright protection and sell the APK by itself. Apple has no power to "kill" off a service, even more so when you consider that said service could easily just also offer a web-page, like Tinder/TikTok/etc. where even if you don't want to sell on Apple's AppStore, you still can reach iOS users just as easily. That is why they are not unrivaled gatekeepers. That's why Parler cannot be "banned" from iOS devices. Yes Apple can prevent the App from being sold through it's AppStore, but they cannot (legally) prevent you from simply visiting the Website and using it's service on your iOS device anyway.
People fail to understand that developers are not entitled to be "able to sell on specific app-stores".