This is a completely made up and hallucinated problem. I will not mince words - this is a blatant attempt at deception.
We do not need to block sideloading to:
1. Stop malicious apps (does nothing)
2. Stop users from side loading
If we want to stop sideloading, we can simply introduce an arduous process to enable side loading. For example - consider turning on ADB. Do we vaporize ADB? No, because that's fucking stupid.
But now when it comes to apps, that little nugget of information is suddenly conveniently not considered.