I think it's important to understand the Payment was deferrable. The tax was still owed. It's the due date that could be manipulated under the temporary provision. This is why people whose tax return ended with a refund found that the deferral was not being honored = because they have enough already paid in to cover that tax owed, there was no deferral function that would result in a refundable overpayment.
"This means that self-employed individuals that defer payment of 50 percent of Social Security tax on their net earnings"
Let's understand that the amount he was entitled to defer is moot for him, because he did not defer. The IRS is right: his safe harbor is 110% of his total tax from the prior year. He did not pay that amount.
Also, I think you are misunderstanding "the payroll tax deferral period" which refers to paying 1040ES for the periods from March 27, 2020 to Dec 31, 2020. That means for 2020 ES, the second, third and fourth quarters, for the most part, can be less than otherwise would have been computed, taking into consider the option to defer will be applicable to these payment periods. It isn't applicable to 2021 safe harbor.
It might help to understand this provision. For the self-employed person, to give them parity to employer/employees, the typical employer Social Security component of a FICA deposit would have been owed and payable. The CARES Act provision meant the employer did not have to make that deposit, and even allowed them to defer collecting for that specified period from the employees' wages.
Nothing here applies to 1040 ES for 2021 safe harbor. Remember that the SE tax is still part of taxes Owed. They just optional were not fully Due.
The Estimate is always based on what we know applies and what we project. If you were shooting for safe harbor, and your taxpayer did not defer, then your taxpayer should have used the full 110%
"Level Up" is a gaming function, not a real life function.