American and South Vietnamese assaults in Cambodia, a country whose neutrality the U.S. claimed to respect, caused massive, unconscionable death and destruction. The Finnish Inquiry Commission referred to the number of deaths as genocidal. According to the Commission, 600,000 Cambodians died out of a population of 7 million and another 2 million people became refugees. Carlyle Thayer, an Australian Indochina specialist, estimated the number of dead at 500,000 of which 50,000 to 60,000 were executions. The CIA estimated that 600,000 had died.

President Richard NIxon, Henry Kissinger, and the Bombing of Cambodia excerpted from the book Lying for Empire How to Commit War Crimes With A Straight Face by David Model Common Courage Press, 2005, paper

If you want further comment, you'll need to track down the original CIA report AA, and for that matter source AND reference Chomsky's original quote, so we can compare the two.

But,as you can see from the above book excerpt-
A Finnish commission also estimated the deaths at 600,000 plus an Australian expert arrived at 500K.
This book also says the CIA estimate was 600,000, and it is not a Chomsky book.

but this all rather trivial methinks, as both casualty estimates were then eclipsed by the subsequent Khmer Rouge- which the bombing radically increased popular support for, as the then Lon Nol government in Cambodia was considered 'pro-US'. The important thing, from the public interest perspective, is that it was principled dissidents like Chomsky and a few others who brought these atrocities to the attention of the public, whilst faced with a barrage of denial from the Nixon administration. They were proved right.