China has built over 200 of the Z-9s and many have been armed (with twin 23mm cannon, torpedoes, anti-tank missiles, and air-to-air missiles). The Z-9WE is the export version which is modified to more easily accept Western electronics and weapons. Cambodia is initially receiving the transport version of the Z-9.
China is the main supplier of military equipment to Cambodia. The U.S. had been ready to supply this material but backed off because of the corruption and mismanagement of the Cambodian government. That sort of thing does not scare off the Chinese. Moreover, the Chinese need some allies in this part of the world because most other nations are uniting against Chinese claims to control the entire South China Sea. Cambodia has no problem with that because it has no seacoast on the South China Sea (just the Gulf of Thailand). Thus Cambodia is the only nation in Southeast Asia that is not hostile to China.