What is the prp-advantage of a $2^{80}$-time adversary attacking AES-256?
You misunderstood what Dan Boneh was trying to say. There is no known attack on AES256 that comes even close to this complexity. However, if there was a known attack that came close to this, then it would be very unwise to assume that this is the best attack. When formalizing an assumption, one needs to be conservative relative to what is known. Thus, Dan was giving an example of how one could formulate a conservative assumption about AES, and he was being very conservative (which is a good thing).