I understand your confusion, but if you look at the descriptions, you will realize that they are different abilities. Kanya+: Chian damage boost (this is only to make the chain complete faster, it does not give more damage) The ring of CoW and Lightning's stm: Chain Damage Limit Boosts (this one does increase your damage) is somewhat confusing, but if you do the test, you will notice the difference in damage.
How. Just how. My team is stacked with NV units with main stats all over 8k minimum and I can barely scratch this dude and this guy does it with a half naked team.