>it'll pass App Store review just fine.

Do you have any evidence of this claim? It's possible that neither Apple or third party developers are able to ship apps through the app store with it.

Why would Apple go through the App Store review process for their first-party apps?

Because it's cheaper to maintain 1 ingestion pipeline than 2.

And it's basically free to create a rule "if it's from apple, auto-approve" even if it's a manual process.

Well the automated parts of the process may still be useful to have the app package run through. Checks like "Does not use private APIs" are important to avoid accidental anticompetitive behavior. When working in large organizations communication is difficult and automatic checks that protect against mistakes are important.