So, if someone is subject to RMDs and received less than $1200 because of ‘19 income levels, and opts to not take a distribution this year, with the resulting lower AGI they will still get some more stimulus money?
Yes, on your 2020 tax return you'll calculate the amount of stimulus due based on this year's income and subtract the amount already received in advance. Any number greater than $0 will be entered as a refundable tax credit (meaning that that even if your tax liability is $0, you will still receive the payment).