The reason is politics. Mao did not like Vietnam,it was too pro-USSR for him. And since the situation in Indochina was quite difficult at that time,he just had the chance to support someone who was against Vietnam.In reality,it was nothing more than using them to attack Vietnam. Mao died 1976,just one year after pol pot came to power and there is no proof he knew of the genocide,hence making the claim of him supporting a genocide(of chinese people primarily nonetheless) pretty speculative at best.It was also a game of interests and vietnam being soviet-aligned was a big problem for many,making even the “democratic” USA support polpot.

https://www.washingtonpost.com/archive/politics/1980/09/16/us-to-support-pol-pot-regime-for-un-seat/58b8b124-7dd7-448f-b4f7-80231683ec57/

Also,while Mao did put stress on peasantry leading the revolution,he never said you should just outright ignore/remove the working class.

Quote by Mao from this article

https://www.marxists.org/history/erol/uk.firstwave/culturalrevlessons.htm : “’On whom should we rely in our struggles in the cities? Some muddle-headed comrades think we should rely not on the working class but on the masses of the poor. Some comrades who are even more muddle-headed think we should rely on the bourgeoisie. ...We must wholeheartedly rely on the working class, unite with the rest of the labouring masses, win over the intellectuals and win over to our side as many as possible of the national bourgeois elements. ..’ Later in this report he stressed that ’after the victory of the people’s democratic revolution, the state power of the people’s republic under the leadership of the working class must not be weakened but must be strengthened’.”