No they are different mechanics. Sunder armor effectively "removes" armor. For everybody. Armor penetration ignores - for YOU ONLY - a percentage of the REMAINING armor.
So why is Sunder Armor still important? Because ArmorPen still has a cap. Even if you have 100% ArP, the boss will have some armor remaining. Sunder Armor drops Boss armor to below cap at which point you can achieve a "lol no armor" state.