Rubbish.
This is a typical disinformation from the Left, who do not want to take responsibility for what their disastrous policies did to the people of Southeast Asia.
Nixon "invaded" Cambodia because the communists were using it as a base of operations against South Viet Nam. The communists did not move in to take advantage of chaos supposedly caused by the United States; the communists were already there.
I heard it repeated just a few weeks ago during the Howard Stern Show, when this smarmy, mal-educated, and completely charmless stand-up comedian repeating the cliched libel that Henry Kissinger was responsible for the unparalleled genocide that unfolded in Cambodia during the reign of terror perpetrated by the Khmer Rouge.
Apparently, the illegal incursions into Cambodian territory of the North Vietnamese, the Ho Chi Minh Trail, the attempts by the USSR to turn the whole of Southeast Asia-from Thailand, to Burma, to the Indochinese peninsula, to Indonesia-into one vast vast collection of Soviet vassal states was something that this nitwit never learned before embarking on a stellar comedy career.
Cambodia did not have any major bases of operation for the NLF, or North Vietnamese Army.