Histology showed coagulative necrosis (preserved architecture of myocardial fibers) with neutrophil infiltration which hinted that the MI was within 24 hours. Most likely cause of death within first 24 hours of MI is arrhythmia. Myocardial rupture would also be visible on gross appearance of the heart, which they described in the stem.
Mottling and softening of anterior wall on autopsy suggests it was not older than 24 hrs. Death from fatal arrhythmia like V-fib most commonly occurs within one day of the MI. That said, once scar has formed in myocardial tissue it, too, can cause arrhythmia.
MI sudden death -> arrhythmia. The order of commonness is arrhythmia > cardiogenic shock > rupture.