It doesn't say that they accounted for possible changes in item quality. Tide detergent claims that their new 80-oz bottle of laundry detergent can wash 64 loads just like the previous 100-oz bottle because it's more concentrated, and I suppose NPR (if they'd retained a sample of the previous product) could have brought that to a chemistry lab to test and verify that claim, but I have no idea how you'd objectively prove that an ounce of salsa had truly remained the same product.
At some point it will be concentrated it'll be powder again ;-)
Laundry detergent is usually priced as cents per load by savvy shoppers. That would factor out smaller doses.
Sure, but they are accounting for size shrinkage which the original poster was saying they didn't.
I don't really know how you can account for quality either. User surveys? Ingredient sourcing? But then again I think this kind of reporting is just a general barometer. Some other comments are pointing to data sources that might do more of this.