It doesn't matter when Thompson's shoulder crossed the finish line, since the shoulder is not part of the torso.
The torso was chosen as the body part for determining when a runner crosses the finish line precisely because torsos were easy to identify/distinguish and there is no ambiguity as to a runner's torso.
The exact location of the border line between the upper arm / shoulder and the “torso” would vary depending on the development of the upper body of an individual athlete and would not therefore be totally consistent. Based on anatomy, we can say that the endpoint of the torso is the outer end / articulation of the collarbone (clavicle).
Although the pelvic area is anatomically part of the torso, for consistency in photo finish judging, it is more practical to define the lower end of the torso as the horizontal cross section of the body through the hip line . . .
The torso was chosen as the body part for determining when a runner crosses the finish line precisely because torsos were easy to identify/distinguish and there is no ambiguity as to a runner's torso.