Let's see if restating this will help:

"took a 3 year deferral"

Did not opt out of reporting the taxable income over three years. It's not a Deferral. It's allowed to be spread over three years or fully reported in the year of distribution.

"on a 401k distribution for 3 years. Should the taxes he paid in 2020 have also be deferred?"

The taxes are just taxes. Taxes for this are neither deferred or spread out. Each year's tax form has the amount owed, and any withholding is what has been prepaid, along with any estimated payments made.

It's different for the SE Tax. For the SE tax, it was all reported in the year owed, and the Payment Option was to put the deferred Tax on a Payment schedule of 1/2 by Dec 31, 2021 and 1/2 by Dec 31, 2022.

We've seen people confuse these provisions.