OK, so I'm 2 years late...
I agree with OP that it only makes sense that Additional Medicare Tax Withholding be included on Form 2210 Line 11. (Of course there's no guarantee that tax forms, regulations, or statutes will make sense.) But since this is a TurboTax forum, it's instructive to see what TurboTax does. For the 2023 tax year I can see that TurboTax does include it. It's transferred from the bottom of Form 8959 to the Tax Payments Worksheet and from there to Form 2210 Line 11, and on Line 11 it's treated like income tax withholding -- by default as if it were paid in four equal quarterly payments.
