We're talking about a guy that went all John Cena on a titan after having his hand gnawed on. Carving into a titan's hand with a blade certainly isn't out of the realm of possibility.
That was kind of the point. Rather than go for the dramatic and cliche dramatic reveal, they go the opposite direction and it ends up like, "Wait, did this dude just casually walk up and admit he's a titan?"
The manga has pretty much already answered all of the major questions you're probably thinking about right now. It's literally getting ready for the final battle.