In the eyes of the IRS, you were "self-employed" - you earned money by working and the start-up company did not assume any responsibility as your employer.
Since the start-up company paid you but did not withhold and pay Social Security and Medicare taxes on your behalf, you are considered to the the "employer". This makes you responsible for the full amount of those taxes.
Depending on the type of work you did, this income would qualify as "Qualified Business Income".
See "Professional Services” Guidance for the QBI Deduction Helps Define Specified Services for additional information.
