TurboTax defaults to using the method that produces the lowest estimate payment amounts. The option to defer SE taxes will affect those estimate amounts (whether you choose to defer or not) if you use any method based on your 2020 tax bill. Since the option is a one-time only situation, TurboTax did not change the default calculation method.
Excerpt from Breaking down Form 1040-ES: Take the tax you paid in the previous year. Calculate 90 percent of the tax you estimate you will owe in the current year. Compare the two, and take the smaller number. (For example, you paid $500 as tax the previous year. This year you estimate you will pay $1,000, and 90 percent of that is $900. The smaller of the two numbers is $500.) Now compare the total of any withholdings and credits you may have to this number.
The one-time only allowable deferral amount is reducing the tax you paid amount above, even though it should not. Because of this, we recommend taxpayers use the other option - 100% of your 2021 taxes. The way it is being calculated is producing too low of an estimate. The estimated tax should be paid in addition to whatever amounts you deferred (or could have deferred) in 2020, but using the default method - a portion of the allowable deferral amounts is being treated as an estimated tax payment for 2021 - but it is not - those are for tax year 2020.
IRS How To Figure Estimated Tax @LBrand
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