This is also mentioned in the ConeOfArc video linked on stackexchange. However, at 4:17 in the video, the speaker shows a sign describing two versions of the event. In the first version, Wispelaere died due to a German shell (not a machine gun). In the second version, he was killed by the explosion of the detonating device after shortening the fuse (“l’explosion du dispositif de mise à feu”; not sure how to translate this exactly).