No, it does not justify the "armor" that most characters/units wear, I agree. I am not arguing that Swan Lady's or Fountain Gargoyle's Armor are in any way accurate.
But it makes this armor less accurate, because, as Far and Call pointed out, it is made to withstand blows, not shots. After the introduction of post-Matchlock firearms the plates were made heavier, and thicker to withstand gunfire, which most of the time they DID, at a severe expense of mobility. The japanese even imported so-called tameshi gusoku, bullet profe metal plates.
The reason the armor died out was not because it was easily penetratable by guns, it was because it became too unwieldy to wear and killed its owner in the most unfortunate scenario.
The Flintlock mechanism was the last nail in the coffin of the metal armor.
As for sixshooters and muskets...
That's Winchester material.
Latest Yuni's gun was modeled after a Smith & Wesson Model 500, a single action revolver that is used for wildlife hunting in africa, so I'd certainly say they have enough fire to gun down a medieval plate.
Edit: As for Eskatia...
Armor breasts are not accurate.