This is a new feature of Google Play and has nothing to do with extensions. Specifically, Google now by default has a feature called "Automatic Protection" for apps, which if turned on injects new code into the AAB before it is converted into APKs to be distributed to users. The code Google injects requires the app be min SDK 21. It looks like if you turn it off you can upload AABs created by App Inventor without issue (like every app ever made up until Google introduced this new mechanism). Here are some screenshots I took of the process:
- Under "Test and release" go to App Integrity
- Locate "Automatic protection"
- Click Settings
- Turn off the "Installer check" option and then click save in the bottom right.
At this point you should be able to upload your AAB without getting the min SDK error.