Slight amendment: 12.4% of the payroll tax stops at the ceiling for FICA, which is $110,100 this year. However, the rate is actually 8.4% this year, due to the temporary reduction in FICA taxes.
The remaining 3.9% is for Medicare, which has no upper limit, at least on wage income.
Neither tax is imposed on unearned income like interest, dividends, and capital gains.
You’re right, of course. A bit of sloppy writing and thinking on my part.