Do GNU/Linux repositories count? F-Droid?
https://en.wikipedia.org/wiki/List_of_Android_app_stores
Really hard to say what would happen in a hypothetical situation where Apple allowed iOS alternative app stores. Maybe the overwhelming majority of users will continue to download only on the App Store, with a tiny minority of power users going to alternatives.
That's absolutely what will happen, there's no evidence of otherwise.
But this is all beside the point. The argument is, if there existed a app store that was as large as Play Store / iOS store, it would have similar moderation issues. The old headline implies that these companies aren't trying to moderate, rather than realizing that moderating content at scale is a REALLY hard problem no one has solved.
No Google/Youtube, not Apple, not Amazon/Twitch, not Facebook, not Twitter, not Microsoft, no one.