No, that's not true. I've heard this quote often, but it's an out of context representation of the situation. The US did not support Pol Pot up to 1975 (he was later granted diplomatic support by the US at the UN when the Vietnamese removed him, but that was something entirely separate, though obviously reprehensible). What is meant by that quote is that the United States' policy towards Cambodia laid the groundwork for Khmer Rouge's radicalism.

Firstly, the US replaced the popular King Sihanouk in 1970 with Lon Nol, a general. Sihanouk was the King of Cambodia at the time and was highly popular among hte Cambodian people. He was battling a civil war against communists, and was taking American support for that conflict, but he was also experimenting with socialist policies and was a neutralist, pursuing good relations with China and the USSR. The US leadership suspected that Sihanouk was playing the US, suggesting that he wasn't fighting the KR as hard as he could have been and that he was essentially taking American money without being as pro-American as he could have been. So he was replaced with Lon Nol, who was corrupt, repressive, and himself quite racist (Lon Nol's policies were strongly anti-Vietnamese, and he slaughtered several thousand of Cambodia's ethnic Vietnamese, a policy which would later be emulated by the KR). Furthermore, the American bombing of the Cambodian countryside (which could have killed upward of 900K+ people) sparked a huge backlash among rural Cambodians. This all came to a head in 1975, when the US pulled out of Indochina: South Vietnam collapsed against North Vietnam and the KR succesfully kicked out Lon Nol's government. FULRO rebels in Vietnam's central highlands, many of them ethnic Khmer, suffered greatly at the hand of the South Vietnamese, and often jumped between the borders of Vietnam and Cambodia, hoping to aid their Khmer brethren and use eastern Cambodia as a touch off base.

Now what's important to note here is that most Cambodians (particularly rural Cambodians, who were the predominant rank and file of the KR) were illiterate and Marxist teachings actually were not particularly popular among many of Cambodia's rural population. However, anti-Americanism and anti-Vietnamese sentiment were and that's the context of this quote: America's irresponsible actions in Cambodia led to the anti-Americanism which created so much of the KRs radicalism.

Additionally, much of the racism which the KR perpetrated against minorities, particularly ethnic Vietnamese, could be linked to the idea of 1. ethnic vietnamese control of the economy (this isn't really related to hte US), 2. and the Lon Nol era's legitimation of racist policies (sadly, Cambodia still today has a significant problem of anti-Vietnamese racism: https://foreignpolicy.com/2017/01/05/cambodia-wants-china-as-its-neighborhood-bully/).

So hte idea is not that the KR were given money or weapons by the US, but rather that they were simply the byproduct of bad decision making and policies.