Just take a good look here -
http://www.onemanga.com/One_Piece/554/02-03/
Yep, it's arms only. I hope now you see how he could be zoan.
Let me remind you. We saw only 3 bird zoans. Pell, Lafitte and Marco. We only saw one Pell's transformation. When you think about it, every time he transformed, the situation required that he transforms fully. One case is hardly enough to conclude that bird zoan's aren't able to transform only their hands, right? Wrong. There were tons of people, like you, thinking that bird zoans can't transform just their hands if they want to.
Then Marco appeared, revealed that he's a bird zoan and he has shown that a bird zoan can transform his hands alone if he wants, disproving the wing paramecia theory.
Why some people still think that Lafitte has paramecia, beats me...