I was a touch surprised at coal. I can see how it would work, its highly carboniferous and really porous.
Thinking back to the clinker paths (perhaps clinker isn't coal-y enough though) that have been ripped up and re-planted, they did seem to be moister than the rest. Whenever I came across coal in the soil, the soil was moist.
However the thing I'd be worried about is the accumulation of heavy metals.
I think the article hints at coal's ability to capture those heavy metals and not release them since biochar has that ability. Basically, since you aren't burning it it won't accumulate. At least that is what I get from the article.
Thinking back to the clinker paths (perhaps clinker isn't coal-y enough though) that have been ripped up and re-planted, they did seem to be moister than the rest. Whenever I came across coal in the soil, the soil was moist.
However the thing I'd be worried about is the accumulation of heavy metals.