I thought that was where the article was heading when it mentioned that horizontal was better (and the unstated fact that mono was horizontal), but it still took me a minute to realize that the 45 degree pickups are equivalent to midside. In (digital) mid-side you encode M=L+R and S=L-R. With this, if you breakdown the horizontal and vertical components:

    HL = +L/sqrt(2)    VL = +L/sqrt(2)   L = sqrt(HL^2 + VL^2)
    HR = +R/sqrt(2)    VR = -R/sqrt(2)   R = sqrt(HR^2 + VR^2)
     H = HL + HR        V = VL + VR
       = (L+R)/sqrt(2)  V = (L-R)/sqrt(2)

Note that VR is negative and HR is positive because the sign of the pickup is inverted in the diagrams.