Per the designer via the article:

This piece is knitted lengthwise from side to side. After knitting several rows, the stitches are divided; the front and back are knit separately and then connected later. The overlap allows for the natural movement of your shoulder.

An optional matching piece, called a “Haramaki” adds extra warmth around your waist and brings a unique look to the vest. Or you could make the vest longer, so it might become more like a tunic!