I appreciate the clarifying question, I actually have an Iranian friend that explained to me that in Farsi, that the slogan 'Marg Bar Ameriki' or 'Death to America' (farsi does not contain 'down with xyz' in it, 'death to xyz' is the linguistic equivalent), it's is not commonly intended as a literal death threat, but an expression of political opposition, or opposition to a state of affairs.

One might say 'death to traffic' for example, that doesn't mean you want to literally kill everyone blocking your car or whatever, it's more a statement of exasperation and opposition. As a political slogan it isn't intending to mean death to Americans, but to the American government. It does sounds edgy in an english translation though.

To your question, 'white' is not a race, it's more a self-identification and class signifier. Non-white ethnicities (Irish, Italians, etc) have a long history of assimilating as 'white' by becoming cops, landlords, and other active participants in American settlement/ westward expansion. That's not to say there aren't poor people who also identify as white, but if they are ever going to really have solidarity with other nonwhite people they really need to kill that white settler in their head.

I read 'death to whites' in this context as a poster complaining about, and expressing opposition to the bias in narratives/social media platforms - the thread was originally someone venting about racist comments on reddit, and white lemmmy users felt the need to get defensive about the most surface level reading.

For reference I said 'death to america' just the other day in a reply to a post about how florida prisons are going to be forcibly detransitioning prisoners, because it's a fucked up situation and at this point I don't see a way to really fix the bigger problems at play without some fundamental changes to what 'America' is.