I noticed this too. I had never heard this before, and the Michigan Hunting Guide confirms that it would not be legal: "Muzzleloading Deer Seasons: An individual hunting deer with a muzzleloading firearm during the muzzleloading deer season shall only possess or carry afield, or take a deer with a crossbow, muzzleloading rifle, muzzleloading shotgun, or black-powder pistol, loaded with black-powder or a commercially manufactured black-powder substitute."
Maybe the poster is from a neighboring state?