From the article:
The model makes a simple call -- whether the equity index will be higher or lower after 30 days -- and over almost four years it’s been right 68 percent of the time.
Don't know about the Nikkei, but the S&P 500 daily movement is up 53.6% and down 46.4% for the period of 1950-2015. So, 68% is indeed a bit better than average.
However, if the 32% that the model is wrong costs you more money than what it makes you when it is right, then it's still no good.
For example, suppose that in the 68% of the time when it is right the market only moves 1% or 2%, but in the remaining 32% when the model is wrong the market moves 10% or more in 30 days, it still costs you money.