I hope i'm not wrong, and someone correct me if i'm. But from what i seen and learned it, they are not the same.
And by that i mean they are not cumulative, they are calculated separately.
For example, if you have a 50% decrease def debuff on the target, and you have a hero with a skill that ignores 50% armor, you wont have 100% armor penetration/reduction. Insted, you'll have 75%.
If the target has 1000 armor, you debuff to reduce it by 50%, to 500 armor, then you ignore 50% of it, will be 250 armor left. If you have Cruelty as well like you asked, will be 20% of that 250, so 200 armor left. so you'll ignore only 80% with all this.
But being able to ignore this much is not bad, the more the better i would say
So you can't ignore full armor unless the skill says it.