I went back through the questions, selected "Yes" (this time assuming they meant 2020) and everything worked out properly on my 1040.
This was extremely frustrating
The instruction is correct. Where TurboTax is asking for your basis in nondeductible traditional IRA contributions made for 2019 and prior years, that's precisely what it wants you to enter, not any basis added by a traditional IRA contribution made for 2020. In essence, TurboTax is asking for the amount from line 14 of your most recent previously filed Form 8606. If the contribution you made in 2020 was for the 2019 tax year, that contribution would have been included on your 2019 Form 8606.
The basis for a traditional IRA contribution made for 2020 is added by entering your 2020 IRA contribution under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions. If you included your contribution made for 2020 when TurboTax asked for your contribution basis from years prior to 2020, that has likely produced an erroneous Form 8606 with your contribution for 2020 being present on line 2 instead of on line 1.
Ok, well I'm glad it makes sense to someone.
I already filed so can't get back to the page in question to better explain what I saw, but I'll do my best to remember. When I first started, I answered "yes" to the question about contributing to a traditional IRA so TurboTax took me to that part of the wizard. I don't remember the exact page where I entered the amount I contributed so I'm not sure if it was the page you mentioned, but I just clicked through the pages TurboTax presented; I didn't get there by jumping around, so whatever I encountered was part of the "default path" through the application. The step that confused me was a yes or no question about whether I had made a non-deductible contribution in 2019 or before. I have been using the backdoor Roth IRA method for a few years, so while I technically had made a non-deductible contribution in previous years, I had also converted it to a Roth within days of the initial contribution so my basis (as I understand it) should have been zero as I always carry over a zero balance in my traditional IRA account. I initially answered "no" thinking that this would lead to a zero basis for the conversion; however, this led to 4a and 4b on my Form 1040 having the same value and the contribution being added to my income for this year when it actually came from my income this year (having been a non-deductible contribution). When I went back through the process and selected "Yes", 4a had the appropriate amount and 4b had the $1 of interest I earned on the money before converting it to the Roth. I'm certainly not a tax expert, but I believe this is the appropriate outcome.
If the 2019 figure is correct then perhaps the instructions on that page could be improved to make the desired outcome more clear. If TurboTax is looking for "line 14 from your most recent Form 8606" that would be very helpful information to provide.
TurboTax does say that, but refers to the line by name, "total basis" rather than by number because the line number was different a few times 15 or 20 years ago. I just refer to line 14 because those doing backdoor Roth have almost certainly filed a Form 8606 more recently than that and it's on line 14.