Terra Preta is noble savage fiction [1] created because charcoal stains the soil quite deep. (Similar to how the OP 'spotlight' is mostly fiction)
Biochar enthusiasts show the staining in soil cutouts after a few years.
I do a lot of biochar (100+ liters a week) but it has mixed results in the scientific journals, meta studies don't show a lot of improvement.
[1] The Amazonian cutting down of forests and burning it for quick release of nutrients works, but unless a third of your babies die you will run out of forest.
Biochar is just the first step in making Terra preta so it's not a surprise that it doesn't work as well as the final product.