I hate to say it, but did you read the article? Saying "Android has a sandbox" to my comment makes no sense in the context of the issue with Fornite's installer. The vulnerability there was specifically that the third-party app store did not run in a sufficiently sandboxed environment, allowing another application to swap out the app after it was downloaded but before it was installed.

So won’t the same people be screaming if Apple puts too tight of restrictions on a third party App Store? If Apple has to approve third party app stores, would people be happy?

I am very confused as to how approval of third party app stores comes into this? The flow I'm thinking of is going to be "you install a third-party app store, which is identical to any other app except it has one additional capability that it takes an IPA and can request that the system install it". Keeping that in mind, is your comment still pertinent? If so, how?