It is a good point. It depends on how regressive is defined. There are many competing arguments here, but two I am aware of are

1. Regressive deals with % of income spent.

2. Regressive deals with an ideal state where those with more excess income contribute more than those with less.

In both of these, I suppose sales tax is regressive if it applies to all items, but only 2 is regressive with sophisticated rebates and untaxed categories.