Oh, for this part: "or carried back to 2020 and reported on your 2020 Form 8915-E."

That's because Amending 2020 is the same tax year as 2020 filing, so you use 2020 tax forms.

Of course, now, they are using a Forever Form 1040X:

"Which Revision To Use

Use the July 2021 revision of Form 1040-X to amend 2019 and later tax returns, until a later revision is issued. Use these instructions with the July 2021 revision of Form 1040-X.

If you are amending a 2018 or earlier tax return, use the prior revision of this form and instructions both dated January 2020. All revisions are available at IRS.gov/Form1040X."