Sorry--I didn't see the "M" next to the article. The gist was, adding up to 20% water can reduce flaring and soot. I'd try to get away with the least water possible, experimenting until I found a sweet spot. As I understand it, soot is unburned carbon, resulting from too little oxygen to fully consume the fuel. Think about an old-fashioned oil lamp with the wick turned up too high. Turn down the wick (reducing the flame size, which is what adding water to alki does), and there is more oxygen relative to the size of flame to fully combust the fuel, hence no soot.
BTW, I've had a membership for a couple months now, and it's been worth every penny. The premium content is very, very good: solid, science-based testing of gear and techniques; thorough, unbiased reviews of new products; and even discounts on gear through the BPL store.