That's a really interesting design, as it looks like it was inspired by metal broaching tools where each of those rings acts like a cutter to slightly enlarge the hole made by the ring before it.
It makes sense that a penetrator would use that technique as it's designed to clear the chip debris, as opposed to trying to deform the bulk hardened material, allowing far easier penetration and perhaps more energy transfer to the inside of the target.
In the diagram behind the tungsten penetrator, only the part of the penetrator that contacts the sabot has rings on it, while the rest of the projectile is drawn as smooth. The rings might have something to do with helping the sabot separate from the projectile correctly or preventing the projectile from slipping out of the sabot before it leaves the barrel.
It makes sense that a penetrator would use that technique as it's designed to clear the chip debris, as opposed to trying to deform the bulk hardened material, allowing far easier penetration and perhaps more energy transfer to the inside of the target.
https://www.iqsdirectory.com/articles/broaching.html