When you file a joint return BOTH parties get identical letters even if it only pertains to one person.
The deferred tax appears as a refundable credit on Schedule 3 line 12e, "Deferral for certain Schedule H or SE filers." It was subtracted from the tax you had to pay with your tax return. You have to pay half the deferred tax by December 31, 2021 (the end of this month), and the remainder by December 31, 2022.
Click this link for more detailed info on Deferred SE Tax Payments.
