Even though 12 blades are less efficient, the article points out that they may have advantages for the bigger system of a whole wind farm, availability of sites, production, transport, control and non-generating time. It's not beyond belief that all those other advantages could counteract the reduced efficiency. You can't really know from intuition unless someone actually studies it.

Another example - commercial aircraft are known to have a relatively inefficient shape, but that's cheaper to produce than the more complex and more aerodynamic alternatives even though they would reduce fuel costs.