According to TurboTax, the "Retirement Savings Contribution Credit" is a non-refundable credit. However, TurboTax adds $400 to my refund for the credit. Is this an error?

The credit reduced your tax liability, resulting in a bigger refund.

"Nonrefundable" simply means that it cannot be used to reduce your income tax liability below zero.  Until your tax liability reaches zero, each dollar that your tax liability is reduced increases your tax refund by a dollar (or decreases your balance due by a dollar).