Amsterdam - Beijing - Hong Kong 144 transit visa in Beijing

Hi I am a Dutch citizen and will go from Amsterdam to Beijing, stay there 4 days and then go to Hong Kong (stay there 3 days then fly back to Amsterdam trough Beijing). In your article it is stated that the route Amsterdam-Beijing-Hong Kong is applicable for a transit visa in Beijing. My airline company keeps saying that its not valid as Hong Kong is a part of China. Do you have any government source where it stated that Hong Kong is regarded as a third region and therefor the route is applicable