The Simpson’s paradox part is factual, and has been known for over a decade courtesy Steve Sailer, who is an immigration restrictionist. A fairer critique of the poster is that they’re unoriginal (rather than that they’re blaming immigrants, which I didn’t see in this post).
> While US has excellent education, and immigrant children in America do better than their counterparts in their countries of origin, they don’t do as well as the modal group of white Americans, so as the composition changes...
it doesn't say the immigrants are to blame, it just strongly implies that this could be a likely reason, furthermore for some reason to be an immigrant is to not be white which certainly was not the case when I immigrated to America.
A more plausible interpretation is that the education system is failing immigrant families currently. This has the benefit of both matching the evidence and not having racist implications.
> courtesy Steve Sailer, who is an immigration restrictionist
That doesn't make this argument credible. Is there any credible source?
> rather than that they’re blaming immigrants, which I didn’t see in this post
We know well that the talking points of racism are formed to create ambiguity and use dog whistles. That goes back half a century and probably much longer. These are the talking points.
I didn’t suggest this makes the argument credible. In fact, I fully expect that it should make one more skeptical of the argument, which is part of the reason I put that in there.