I'm far from being an expert here but I think I remember reading that the grooves on a "rifle" slug are only there to help a slug exit a tight choke and that they produce little or no spin downrange.
Let me first state that I too am no expert. I use to hunt on several clubs that were "shotgun only" in MS. The State didn't require this, it was a club based rule because they were hunting thickets. Some of the stands were as close as 50 yards and you would never even know someone else was hunting that close to you. I would like to read that article because the rifled slug didn't come out but just around 10 to 15 years ago. Before that, most all the shotgun slug shells were loaded with what looked like the "Buffalo Bullets" we use to shoot in our side lock ML. They call them rifled slugs because of this reason. They have little effect on the projectile but the thinking was, "a little is better than none". It states right on the side of the box of rifled slug shells, "do not shoot through any choke restriction greater than modified, improved chokes work best". (These may not be the exact words, but along those lines.) So, tight chokes are out of the question. It would probably be a sure way to shred the end of your barrel. If not, it would surely do away with any accuracy you may have had.
Then they came out with the rifled choke tubes. You couldn't shoot rifled slugs through these but you could shoot the "old style" slugs with smooth sides because these would benefit from the rifling in the choke tube. Right after that was introduced, the rifled slug replacement barrels hit the market. It was a natural progression of finding ways to make the slug guns more accurate.