“Not a clone, not an imposter, not mind control, not someone else” does not include “no time travel shenanigans involving a cosmic cube/Kang/Doom’s time machine/Mephisto’s powers”, so I’m willing to bet that the same bright ideas that brought us “One More Day” are responsible for this new twist. Oh well, in a few months this will be re-retconned anyway.

Look, there are a couple of possibilities here:

1. Nick Spencer is lying because the company line is following some JJ Abrams-style mystery box plan to gin up controversy and thus sales. This isn’t a good way to treat your customers. You get more money in the short run, but you’ll also lose some fans in the short run, and you run the risk of damaging a very valuable brand. 
2. It’s time travel or cosmic cube or whatever. As I said, that’s still bad writing. It’s not the kind of twist that adds rather than detracts from the character, it doesn’t suit the character, and it’s going to lead to clumsier writing to fix it. 

Whichever one is the case, this run is going to be remembered as “the one where Cap’s a Nazi,” and that’s terrible.