we are making a very strong claim when we say that slavery is a direct result of lower IQ.

I meant the views on black people.

Secondly, your objection now reduces to "other people in history were predjudiced against blacks, so modern prejudice is probably not a consequence of slavery". Obviously it reduces the probability, but by a very small amount.

True, we better evidence that modern "prejudice" against blacks is due to the "prejudices" largely being accurate. Namely the fact that the prejudices are in fact accurate, in the sense that (whether because of nature or nurture) blacks are in fact less intelligent and more prone to criminality than whites.

Are you seriously trying to argue that the prejudice against blacks in Europe and the USA is not a consequence of the slave trade?

They are an indirect consequence of the slave trade in the sense that the slave trade resulted in large numbers of blacks in the United States (and also possibly contributed to the difference in intelligence).