Yes, it's a serious question.
A sales tax is not regressive because it is applied equally to all purchases. Those with very low income are taxed at one percentage, and those with very high income are taxed at the exact same percentage. That’s flat.
I know the argument is that sales taxes are regressive because if you are paying taxes on necessities, then a higher percentage of your income is taxed. This has nothing to do with tax rates being progressive or regressive. Those are defined by rates, not by total amounts. If we are to believe that sales taxes are regressive, then by definition we must accept that flat taxes are highly progressive (a contradiction).
Additionally, this is a very weak argument in general. Those with lower income pay a higher percentage of their income in general. Screw taxes - rent, food, clothing, car payments, insurance, you name it - EVERYTHING lower income people spend money on requires a higher percentage of their income. That would mean that the price of goods and services is regressive in and of itself, which is completely illogical.