NGC changed the designation to "proof" after they visited China and had a discussion with mint officials, so I'm told. The change occurred after I knew they had returned from a visit to China, so I believe that is the correct explanation.
Chinese government decrees don't always make sense from the outside, so I'm not sure why the designation was unclear at first, and then changed to specifically "proof" while some of the other coins are designated differently.
From what I've seen in their manufacturing quality for the 1999's, they were very poorly produced, and would not be considered proofs in any other minting facility anywhere in the world that I know of. Only a bit more than half of them get a 69 grade, and I've seen a lot that are much lower. They're often quite damaged, probably from the handling done in the gold plating process, if the plating was done AFTER striking, which I'm not sure of, but I think is likely.