No, this is not business income. The employer made a mistake by not including this income on your W-2.
Enter the Form 1099-NEC under Income from Form 1099-NEC, not under Business Items. Do not link it to a self-employment business. TurboTax will pick this Form 1099-NEC up to report on Form 8919 when on the Does one of these uncommon situations apply page you select "My employer reported this extra money on a 1099-NEC but it should have been reported on a W-2." Do not enter it enter it on the directly on the Firm and Payment Entry Smart Worksheet. You should be doing all of this in step-by-step mode, not in forms mode.
The IRS (or maybe the Social Security Administration) will go after the employer for the employer portion of Social Security and Medicare taxes that the employer failed to pay.
