It seems a bit of a guess that exact number. But to my understanding what he’s saying is that the angle of vanishing stability means that they yacht WITHOUT any external forces acting on it would right itself up to a heel angle of 73°. However, if you include the heeling moment of the wind, the heeling moment will overpower the residual righting moment much before the angle of vanishing stability, which he guesses to be about 2/3 of the value of AVS, hence 50°.