Apparently it does cool the atmosphere though, so it's good against global warming.
In case it's not: particulates do indeed cool the Earth. This mitigates the warming effect of CO2 emissions to an extent. The problem is that particulates don't last very long in the atmosphere, whereas CO2 lasts centuries.
To maintain a constant level of cooling, you have to keep burning more coal, which steadily increases the atmospheric concentration of CO2, which steadily increases warming. Before long you'll have overcome the cooling effect entirely and will be on a steady path to increased warmth.
Whatsupwiththat.com is a notorious climate change denial site, and the second sentence in the Wikipedia page you linked says:
This hypothesis had little support in the scientific community, but gained temporary popular attention due to a combination of a slight downward trend of temperatures from the 1940s to the early 1970s and press reports that did not accurately reflect the full scope of the scientific climate literature, which showed a larger and faster-growing body of literature projecting future warming due to greenhouse gas emissions.