I believe the idea is to note that European settlers already had contact with another indigenous group and had knowledge of what their own presence could do in terms of devastating a population. Similar dynamics took place there, including the need to collect humans for slavery and the desire to Christianize a pagan population. The Spanish and Portuguese saw how diseases wiped them out, and rather than desist in trade, they took efforts to conquer the newly depopulated land. So to take the position that the European explorers who went to the Americas didn't know what they were doing to the Native American population because they had no working idea of immunity to disease is not tenable, because they did in fact have experience already with such contact.