If one takes the position that we do not have enough evidence to come to a particular conclusion (as Lang did), then one takes that position with respect to the information available at a particular time. I don't know at what point Lang last stated such a position. Regardless, he's been dead for nearly 9 years.

AIDS is one of the most studied diseases in all of history; we learn more about it every year. It is certainly possible to state that we know HIV causes AIDS, without disagreeing with with Lang.