Gold Palm, a consultancy no-one has heard of based in Shanghai. I am sure numbers have dropped, but not by 50%. Otherwise we would hear much more about it and HK government figures would not state a decrease of 1.9% year/year in June.
Being generous, perhaps Gold Palm's statistics are based on tourists from Shanghai only.
The negative opinion of many mainlanders towards HK is real though. Not scientific, but The impression I get from my mainland friends here, who say their friends back home are increasingly thinking that way.