The US involvement in Vietnam would have already been over before the uprising of Pot had Nixon and Kissinger not skuttled peace talks to help Nixon get elected.
See a discussion of “proximate cause.”[1]
[1] https://legalclarity.org/what-is-the-difference-between-dire...
The Khmer Rouge and other opposition groups were fighting the government in an armed conflict since 1950. The Khmer Rouge controlled almost half of Cambodia’s territory 10 years before the bombing.
In the grand scheme of Cambodias civil war, the US bombing didn’t play that big a role at all.
The ideological content of the Khmer Rouge is very poor overall, and is a mix between nationalism and communism aiming to "clean" the nation from western influences. Which is kind of ironic given Pol Pot's background.
[1] https://en.wikipedia.org/wiki/Pol_Pot