Yes, basically.
"suit of armour" is countable. You would wear a suit of armour but not an armour. It's like having a glass of water instead of a water or a piece of luggage instead of a luggae.
I agree that it sounds a little confusing.If a countable noun (suit, glass, piece) goes first then there's nothing in the sentence to tell you that the uncountable noun is uncountable. This can be a bit tricky, I agree.