I can see how it's armored, but they still look way thinner than his fingers in his armor ever looked before. That's why in theaters I thought it was just his hand.
I wouldnt be surprised if it most likely his finger and in post was digitized to look like metal.
There's many behind the scenes videos where Downey is most of the time only wearing partial parts of the suits, a the whole suit was digitally created on post.