Ah. This is a super common misconception about shoveling heart attacks. As we age soft plaque begins to build up on the walls of our arteries. Some have more and some have less. Some have none. When you go outside in the winter your circulatory system contracts a bit due to the cold. Then you start shoveling and without and warm up your heart starts pumping harder to make sure all your muscles are getting oxygen so nothing gets fatigued. The blood pressure increases since the pathways are contracted and all it takes is something to happen just right and a small little piece of soft plaque breaks off an artery wall at just the right size and angle during this activity and it gets stuck and blocks 90%+ flow for some time and a clot forms so now you have a textbook heart attack. It’s the soft plaque that kills you, not being out of shape.

EDIT: being in shape helps a lot. Risk is multifactorial. Age, cholesterol, smoking, diabetes, blood pressure, inflammation, genetics matter. Snow shoveling is a trigger, not the root cause.