The IRS says the 1040X efile will be available “this summer” but no news on definition of “summer”:) So yes the return must be mailed. They should mail by 7/15 to avoid penalties if there is a balance due.
Do not use 1040X. The IRS stated in a press release
<span style=”font-family: times new roman,times; font-size: medium;”>Duplicate returns related to Economic Impact Payment (EIP): If a filer used the Non-Filers tool to register for EIP, they cannot file a subsequent tax return electronically. They must complete a paper Tax Year 2019 Form(s) 1040/1040-SR, print and write “Amended EIP Return” at the top, and mail the return to the IRS. The tax return must be mailed by the postponed due date of July 15, 2020 to avoid late-filing penalties and late fees.</span>