It is regressive when you consider the position of the buyer. People need to buy large items, and low income people are disproportionately hurt by taxes on those items.
That doesn't mean it's regressive. It's just a demagogy.
Low income people buy less things and pay lower sales tax proportionally. Wealthy people spend more on luxurious goods and pay way higher tax in absolute numbers. Everyone is hurt the same.
On top of that, necessities (food, medication, books) are usually exempt from the sales tax.
The logic of calling a flat sales tax regressive is (I think) that poor people spend most of their income, while rich people save more, and thus pay a lower percentage of their income.
Agree that this isn't a great way to look at things. And that (as you say) there are often different rates of sales tax on potatoes vs yachts.