In terms of the truth spectrum, IGN can be right, wrong and mislead (developers did tell them something but those developers were wrong), wrong because they misunderstood (developers told them something and the paraphrase changed the meaning, like "broken telephone"), wrong because they were lied to (developers lied to them), or bullshitting (making up).
With the 3DS = PS360 power level, if you look at their actual claim, it seems likely that they're either right, mislead, or misunderstood.
With Factor 5's Kid Icarus, it was one part misunderstood (they misinterpreted F5 pitching KI as being F5 working on KI), and one part bullshitting (any time they implied it was in an advanced stage of development or just about to be revealed).
With this, they're pretty much either right or they're bullshitting. We know Retro's project is well underway, so they can't be mistaking a pitch for a project. They're not going to be mislead or lied to on this. So either they've invented this out of whole cloth, or they didn't, they actually were told Retro was working on Donkey Kong, and they're right.