I think they're referring to the radiation emitted because of nuclear plants, like nuclear accidents at Marcoule, Fukishima, Paks, etc, transport and waste disposal and so on.
"As a general clarification, ounce for ounce, coal ash released from a power plant delivers more radiation than nuclear waste shielded via water or dry cask storage." (emphasis mine)
By adding the qualification "shielded" the comparison becomes meaningless. Brazil nuts, bananas, beer, carrot juice and drinking water [2] all release more radiation than shielded nuclear waste.
Nowhere in the SA article they compare coal ash with Chernobyl of Fukushima.
[1] https://www.scientificamerican.com/article/coal-ash-is-more-...
[2] https://www.thoughtco.com/common-naturally-radioactive-foods...