it seems like the implication is that if Americans aren't constantly ripping our hair out and rending our clothes over the genocide of the American Indians, we have no right to decry modern genocides, or something?

Yeah, but the issue isn't that they aren't ripping their hair out and decrying the crimes of the American empire, it's that these crimes are being presented as something in the past and not something ongoing (which they are).

The implication that killing "the other" is less bad than killing "your own" is also a major dog whistle.

You also need to approach, specifically in this instance, mass death from famine not from a position of "this was an intentional genocide", but from the historical perspective of the pain and suffering caused by transition from agrarian feudal society to industrial society.

The famines in Russia and China are always treated with a different pair of gloves compared to the famines caused by American industrialization (mostly pushed onto the native population and the stealing of land was used to feed a growing industrial proletariat), British industrialization (the Irish potato famine and at least a half dozen Indian famines that pushed the grain shortages on to "the other" to protect their own industrial proletariat).

Another thing to pay attention to is always the response to these famines within the context of a given socio political order. Both Manifest Destiny and British colonialism ignored the effects of the famine because the people it primarily effected were enemies of the imperial cores. Whereas both Russia and China experienced famines that harmed their own industrial output and forced them to reconcile the massive issues within their political structures. To the point that famines which used to be common were all but eradicated after massive political pressure to fix the issues that caused them.

The green revolution has given us ground to stand on as productive output per labor hour in the agricultural sector has massively improved and allowed for a majority of labor to be in industrial and service sectors, but now that we're experiencing the effects of climate change we're gonna start seeing the conditions for mass famine re appear.

And I guarantee that the US empire and Western powers will do what they always do and force the hand of their neo-colonies to absorb the mass suffering caused by grain shortage to maintain the proletariat in the core.

This isn't something that's "in the past", it's still standard operating policy all across the Western world. To siphon land and grain from imperial subjects and cause destitution and starvation to maintain profitable industry for their domestic bourgeoisie.