In the case of Cambodia, I think the connection being suggested is that the genocide targeted "professionals and intellectuals"[0], essentially the bourgeoisie, or the wealthy (it was a communist revolution). So criticizing inequality is being framed as a slippery slope to killing rich people.

[0] https://en.wikipedia.org/wiki/Killing_Fields

Well, I'd argue that the gap between making racist remarks and committing greater acts of racism is a bit smaller than the one between criticizing inequality and emulating the mass genocide of a communist regime. Although the other OP did bring up the Nazis, so I see where he was coming from.

I think they might be alluding to things like "kill all landlords" which are I suppose jokes for some and simply calls to action for others.

I'd love to see an example of a landlord being killed as a result of such a "call to action" from a political movement.

I can point to numerous people being killed by calls to action around abortion.

A few hundred-thousand dead Kulaks is a good example. Killed because they were just above the peasantry.

https://en.m.wikipedia.org/wiki/Dekulakization

And how many were killed because they were on a funny name list?