Does the fact that African immigrants are a highly successful demographic prove that systemic racism doesn't exist?

This is an argument I come across often so I’m curious to hear your explanations for why it is not true (if you regard it not to be). Immigrants from Africa are apparently a quite successful demographic in the United States, far more so than African Americans. I’ve heard this used as evidence that the reason African Americans are disproportionately poor is cultural and not the result of racism.