Because Channeling while blocking will negate all incoming damage (rendering the Armor increase irrelevant), the effect is only present when Channeling but not blocking. Even with the highest possible Armor before Abilities (a Valkyr with Steel Fiber and Armored Agility, for a damage resistance of 83.61%), a 20% increase to Armor only results in a 2.35% increase to damage resistance. Blocking but not Channeling with any melee weapon will reduce damage by a significantly greater amount (the lowest damage block of all melee weapons is 35%).
Mod currently does not boost Rhino's Iron Skin nor did it seem to reduce damage from a level 100 Ballista during a test in the Simulacrum. Either the increase in armor is extremely small (too small to register a decimal point in Iron Skin) or the mod is currently broken and not working as intended.