Google Play Dumps APKs for the More Google-Controlled “Android App Bundle”

>App Bundles spit out APKs. That means an entire app build system must be hosted in the cloud.

Why? Do they really mean any system or just Google's system with Google's requirement to assemble APKs on their side?

All these App Bundle articles are confusing in this way. What is required by Google, and what is required by future Android changes?