Means it's not cannonical to the series, in other words, it shouldn't be taken is a real part of the story in general.
Capcom are going to great lengths to shut out DMC 2 because of how big a failure it was, it wasn't made by the same director who made the other games, therefore, it wasn't in the same style and it flopped out big style, so, as far as Capcom are concerned, it no longers remains a part of the official DMC storyline.