(Note: For the sake of simplicity, assume that this relates to someone who is entitled to receive (and keep) the full stimulus amount.)

The money that is being sent out relates to a federal action taken in 2020, which they need to put on the books, so to speak. They're going to do that through the income tax filing process for 2020, which actually takes place next year.

So, a tax credit will need to be shown for this item when you do your 2020 tax return, that you file in April 2021, because it was revenue that you received that year. But since you already received this specific cash in 2020, this new tax credit won't generate any more cash being sent to you in 2021 to settle up your 2020 tax return. They don't want people surprised by that. I think that's all this means.

FYI As far as I can tell from looking the tweet referenced, those screenshots are not coming from any federal government documentation. I saw at least 2 different sources being suggested in the tweet replies. So the paraphrasing by the source may be the problem.