It's kind of what the whole article is about...

It's an estimation based on the fact that yields have exploaded, soils contain less nutrients on average, and foods are consumed less fresh which gives them time to lose nutrients.

There are plenty of reputable sources on each of these factors, so I encourage you to do your own research instead of taking me on my word.

And I'm not saying every food you can find today are 80-90% less nutrient dense.

Perhaps it would have been more accurate to say that most modern foods are likely 50-95% less nutrient dense...

The sentiment is the same