Uh, how can he tell it was made fertile hundreds of years ago? Do the years since then have nothing to do with its fertility?
He could be referring to the massive use of fires to change the landscape and irrigation and canal systems established by some of the Indians for agricultural improvement.
Interestingly, the many recent attempts to recreate terra preta have not been successful. Most soils are made fertile with the addition of human waste (cf China). However, terra preta contains many other ingredients which, unlike other man made or naturally occurring soils, does not lose its fertility with years of use, nor does its depth deplete.
If someone finds a way to duplicate terra preta, they will pretty much become instant billionaires, as any soil anywhere could be made to grow anything indefinitely.