Not being able to speak for someone implies you do not know their thoughts or motivations.

So in the second sentence, you acknowledge that you do not know the thoughts or motivations of the person you are responding to, but the preceding sentence implies that you do know the thoughts and motivations of some other people, and what they would or would not have done a couple hundred years ago.

I.e. a very strong claim about one group of people you do not know, followed by an acknowledgement that said strong claim cannot be made for another group of people you do not know.

I don't know the thoughts and motivations of individuals but I can make generalizations about groups. The contradiction you imagined doesn't exist.