Rooster Teeth are the people behind that Halo parody Red vs. Blue and the faux anime RWBY. They are also now partnered with Screwattack to help produce Death Battle, aka, the "Let's create a shitstorm by pitting two popular game/anime/comic characters against each other and explain why one can kill the other" show.
http://www.animenewsnetwork.com/news/2017-06-17/attack-on-titan-anime-season-3-to-air-in-2018/.117628
Good. We don't have to wait FOUR YEARS this time.
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?"