We both agree that Sanders is not necessarily the best — and certainly not the only — representative of these ideas. But your article uses Sanders as an example of someone who understood the connection between racism and economic struggles — even if he did not defend this understanding well — this article went on to talk about (and used Clinton as an example of someone who does not). Therefore, it shouldn’t be surprising when people use his name as a synedoche.
As for the Greenwald quote, even if one acknowledges that there was discrimination existed prior to the scapegoating or that the prejudice originated elsewhere, it’s not possible to exploit that bigotry without also fomenting it. It’s not possible to scapegoat a group without encouraging & normalizing discrimination against that group in ways which persist regardless of whether the problem being blamed upon that group remains or not. So saying that the far right exploits racism as a scapegoat for economic inequality is a statement which imputes at least partial causality — ie, it argues that economic inequality causes racism to be more prevalent than it otherwise would be. Thus the argument that classism exacerbates racism produces the same problematic “class first” attitude as arguing that classism causes racism does.
(Here I should note that I’d already read the article you linked, and it doesn’t debunk anything I said about white people prioritizing the maintenance of white privilege over other interests. In fact, it’s silly to suggest this is at all debatable since it wasn’t until very recently that most white businesses refused to serve black customers — which is about as obvious an example of white people prioritizing racial hierarchy over their economic interests as can be.)