The first two paragraphs deal with Brazilian slavery. “Washed ashore” is just a euphemism for slaves landing on Brazilian shores in slave ships. The author’s meaning is clear to me.
His use of language is good and it’s more colorful and interesting than “Brazil imported millions of slaves” (which is what I would have written in my pedantic and pedestrian style).
“Washed ashore” implies something random, not willful. I’ll go with pedantic & pedestrian when it is truthful. ;-)