I looked it up, and you're right. What was stuck in my head was those striations are called "rifling."
The Foster slug's fins are there to reduce the bearing surface and therefore reduce friction on the barrel, and secondarily allow a certain amount of fire-forming if they encounter a choke. The Foster slug was built to work like a badminton shuttlecock with the center of gravity in the nose.
I've shot a bunch of slugs in a rifled barrel that weren't sabotted, and never had any leading. However, I always made sure they were meant for rifled barrels.