Why copper sheathing was not armor

Did copper sheathing on sailing ships provide any meaningful armor protection?

No. The copper was a thin covering on the underwater hull, not armor intended to resist gunfire.

Copper sheathing covered the bottom of a wooden hull rather than the ship's fighting surfaces. It was too thin to offer meaningful resistance to contemporary cannon fire. At most, a diving shot might incidentally meet the copper below the waterline. The exchange distinguishes a maintenance material from deliberate armor protection.