He has a similar name, he has Obito's eye, and he knows Kakashi and Gai personally. There is literally no other option at this point, barring something silly like Izuna possessing Obito's body and gaining his knowledge (which frankly can't be ruled out at this point, as no option makes sense and is coherent for the timeline). Also, the statement "Tobi has yet to use a jutsu" is laughably stupid.