Are you confident are you about Turbotax protecting user data?

The amount of the credit is the same amount I paid in taxes for the filing year 2020. Only Turbox and the IRS would have that information.

Actually, TurboTax does not have "access" to your tax information. At least, not in a way they can access it legally. If you used the online version of TurboTax, then any tax returns you filed with the online version are stored on the TTX servers, But you, and only you can access it. For anyone else to access your tax returns, including TurboTax employees, is a federal crime.

Of course, that doesn't mean it couldn't happen, and also does not mean a 3rd party could have not accessed it without the knowledge of you or TurboTax. Personally, I have an issue with my tax returns being stored on a privately owned computer that I don't have physical access to, and physical control of. That's why I continue to use the CD version of the program.

would seem to be a scam because if you had an overpayment the IRS would normally just refund it.  on the other hand, if there's a credit you could have claimed but didn't the  only way to get it is to file an amended return. your options to check this are to call the IRS or possibly view your 2020 transcript

It's not a scam. Many people are getting those CP80 letters. It seems that the IRS just sent out a big batch of them. The problem is that the IRS is severely backlogged in processing 2020 tax returns. They deposited your payment and recorded it when you sent it, but they are so far behind that they haven't processed your 2020 tax return yet. So their computerized records make it look like you made a payment and didn't file a tax return.

Probably your best course of action is to just do what the CP80 notice says and send a complete new signed copy of your 2020 tax return to the address in the notice. The IRS will eventually straighten it out.