Oh, right, thank you! The coat does bear some similarity except for those black buttons and what looks to be a zip closure. So I obviously had the wrong event in mind where she wore the red and black outfit; can't think what that was.
I would make the guess that Reiss thinks it's good publicity to reveal what Kate bought, so people will buy the product out.
I suppose they don't care about releasing info for higher sales and will take the chance that Kate won't buy from them if they won't be confidential about her purchases. But perhaps they're confident that she'll continue to buy their merchandise since it's a favorite label of hers.