Where did you get this odd notion that psychologists/lecturers in psychology look at textbooks as these infallible canons of the profession? They are useful tools but it's not like anyone should be using a textbook as a primary source of research. And I'm not surprised a lecturer wouldn't know every detail of a general/introductory type course, my undergrad courses like that covered so much information some of it was bound to be a little wrong. There's a good chance I could things wrong in a lecture on my own area of research if I wasn't reading straight from my sources. Just make a correction politely and don't act smug and like you're cleverly challenging the foundations of psychology when you're making minor corrections to introductory things.