I think you are interpreting the comment too literally. The point is just this: calculating inflation is an art and depending on what kinds of assumptions you make, the results will vary wildly.

Before the printing press, very few people in Europe owned even a single book. But even a lower class, modern European might easily own several dozen books. Depending on how you account for this, you might conclude that the given lower class, modern person is among the richest people in Europe in 1400. Or you might not properly account for the wealth of a 1400 European noble and rank them as middle class by modern standards.

It's simpler with commodities like a bushel of wheat, but still complicated. Depending on what you are trying to explain, you can use different methods but there is not straightforward way to convert the cost of something in one time period to another time period.