No, it's not true. You must enter Cambodia via one of the following ports of entry:
International Airport of Phnom Penh
Siem Reap International Airport is located in Siem Reap, Cambodia.
Pet Poi (Banteay Meanchey)
BavetBavet (Svay Rieng)
Cham Yeam—Cham Yeam (Koh Kong)
Border Post of TropaengKreal (Stung Treng)
International Airport of Sihanoukville