Killian declaring that he's the real Mandarin is the director's way of blatantly spelling it out for the folks who still can't seem to get it. The fact that some people are still in denial because they feel butt-hurt about the twist is just ridiculous and laughable.
Even so, Killian didn't need to say he's the Mandarin. He acted like the Mandarin, he fought hand-to-hand like the Mandarin and he even concocted his plans for a goal like the Mandarin would. Hell if the suited version wasn't enough, they even bothered to slap some dragon tattoos to hint at Killian's fondness for Asian motif and aesthetics (he didn't create the idealized Eastern image of the Mandarin as his proxy for nothing after all).
Also, Shane Black
already confirmed that they actually gave the audience the MCU's version of the Mandarin through Guy Pearce's character Aldrich Killian. Whatever opinions people may have about the twist, it doesn't change the fact that Killian IS the Mandarin in the MCU. It doesn't get any more simpler than that.
Really? Do we really want a new "Mandarin" that was initially some disgraced British actor that got spoiled with money and hookers, yet somehow had an epiphany to embody the "Mandarin" just because he was given the opportunity to act out a created role as provided by Killian (the REAL Mandarin)?
I'm sorry, Ben Kingsley is a great actor, but turning Trevor into the "real" Mandarin is the worst thing they could possibly do. Also creating another "real" Mandarin for the sake of mirroring the older iterations of the character completely defeats the point of IM3. The whole idea behind the film was that you "create your own demons". Those personal "demons" aren't just outside forces (like Trevor's Mandarin portrayal), they're things that you had a hand in molding them, essentially creating your own chief tormentor. By designating Killian as Tony's personal "demon", it further cements his role as Iron Man's true arch-nemesis: The Mandarin