For this part: "I doubt the 2019 AGI was in the picture."

It is not part of the 2020 tax filing reconciliation. It was used to project what a taxpayer's 2020 might look like for purposes of EIP prepayment, if the 2019 tax form was available at the time the EIP payments were being made. Otherwise, 2018 would have been used.

This is not like the EIC, where you can choose which year's AGI to base it on. The Recovery Rebates are based only on 2020 actuals. That's why you use what they got in 2020, and ignore how that was computed, now that you are working on the 2020 tax returns.

There is no doubt. It's in the CARES Act.