Because not tanking the strongest holy sword point blank means you obviously can't tank NPs. That's like saying something isn't waterproof because they can't survive the deepest depths of the ocean.
I don't know just how durable the armor is, but I read that his armor used to tank high level divine beings. I'd bring evidence, but I'm on a phone.
As for Mercury, I don't think he'd destroy the world in 5 minutes. I seem to be underestimating types, and you seem to be underestimating Excalibur. Why don't we just check this out first before deciding who's right?