There are a couple of reasons I could believe this to be an admission;
first; we know Abrams was attempting to cast Del Toro in the role of Khan, do you actually think they would throw out that script which took them so long to come up with just because they couldn't get Del Toro to play the role?
second: Khan makes a much better villain than Mitchell, or Trelaine, or Harry Mudd.
third: its the second film, so why not do an ode to the second film, in the first run of Star Trek films.
What makes me think he might not be playing Khan is his basic look in the shots that have made it online. He looks nothing like the TV or film version of Khan. He's not bulked up like he should be if he is somekind of superhuman. Of course, since this is a re-imagining of the Star trek universe in a different timeline, why wouldn't he look different? Don't the Klingons and Romulans look very different from their traditional Tv and movie appearances?