Why?
I think if you look at the number of years the market ends up, versus the number of years it ends down, one might say the opposite. That combined with the fact that the election is at the end of the year, and the new President does not take office until the following year makes the statistic largely meaningless.
Now if they compared market gains in election years verses market gains in non-election years it might be worth looking at.
The market usually ends up, that it usually ends up in election years is not surprising.