At the time, the US was invading Vietnam, Cambodia, and Laos. China at the time (1970-75) viewed the Khmer Rouge the same as the Vietnamese communists, and offered aid and support in both their struggles against Western imperialism.

This sentiment is expressed by Mao and Zhuo as follows:

https://www.marxists.org/reference/archive/mao/selected-works/volume-9/mswv9_86.htm

https://www.marxists.org/reference/archive/zhou-enlai/1972/03/19.htm

It seems like they were aware of the formation of a ‘third way’ within the anti-imperialists.

What calls for special attention is that at the present time certain powers are creating a so-called “Khmer third force” to carry out activities for a compromise in a vain attempt to split the National United Front of Cambodia and undermine the militant unity of the Cambodian people and the Indochinese peoples

But I don’t think they recognized it as a nationalist movement at the time.

Also nobody mentions that the Khmer Rouge was supported by the Vietnamese as well, at the time.

https://en.wikipedia.org/wiki/Cambodian_Civil_War

If there were any questions as to the nature of Khmer Rouge, any Marxist should be able to critique Pol Pot’s ideology and understand it to be reactionary:

https://www.marxists.org/archive/pol-pot/1977/september-29.htm

Kampuchea: The Revolution Rescued and The China–Cambodia–Vietnam Triangle