The article is confusing. More clearly: both Hong Kong and the PRC (mainland) can deny extraditions from HK to the US.

Beijing, which gave its consent for Hong Kong to sign the agreement, also has a right of veto if it believes the surrender of a fugitive would harm the "defence, foreign affairs or essential public interest or policy'' of the People's Republic of China. In short, the treaty makes Snowden's fate a matter of political expediency not just in Hong Kong but in Beijing.

http://www.guardian.co.uk/world/2013/jun/09/edward-snowden-h...