I don't think publishers/platforms holders, in their PR, have ever quoted competitor figures, even though they have the report that states how much their numbers are. ( correct me if I'm wrong )
Sony/MS/Nintendo are all entitled to reveal their own product's figures as they wish, which will be more accurate than NPD. However, NPD works as a citation benchmark that proves their position in the market, since NPD validates their respective PR statements. Sony could've easily, 2 weeks back, said that they won September, but without NPD's analysis and report backing them, it's just hot air statement.