If it has indeed happened at Sony's request, then it was a good move from them. It must be pretty demoralizing to try to do positive PR about a product when everyone can see how your actual monthly sales are suffering.
I think that if NPD would have released disappointing november sales for the PS3 then a lot of people would decide to delay their purchase from the holiday season to next spring or even later. Even if only hardcore games and such come to these forums, pretty much all media sites do have headlines about the sales data so many potential customers get informed about it.
November sales wouldn't show up until Dec 13. I doubt that NPD numbers for November released that late would affect sales to the point of causing poor holiday sales.