He didn't speak in favor of it. Sources came up saying millions were being killed in Cambodia. Chosmky questioned the sources saying "where the fuck is this coming from? Nothing supports this". Thats his version of the story, yeah but hardly any evidence he supported Cambodia.
Nothing here says he's a fascist. His "genocide denial" stance stems mostly from the idea of being anti-capitalist and not trusting US-centric sources. Its not entirely without merit. Noam Chomsky for example is accused of the same thing for the Pol Pot genocide, though at the time he was right for the same reasons -- accusations started flying in the US based on completely banal sources.
I used to be like this, its not entirely harmful. And in any case, I don't give a shit about his politics. We should be thinking about how to separate peoples politics from the platform, and the work hes done on Lemmy does exactly that.