The App Store review guidelines prohibit this very clearly:
> Apps should be self-contained in their bundles, and may not [...] download, install, or execute code which introduces or changes features or functionality of the app [...]
There is only one narrow exception which clearly does not apply to Roblox for several reasons; for example it's only for HTML5 content.
Tim Cook just testified to Congress that all developers are treated equally, but this seems like a clear case where an app that is "too big to fail" gets special treatment.