I had a similar thought--"Harry cracks magic to the point where people can take arbitrary forms"--and actually, I'd put the probability at >50%. In fact, I am slightly worried Eliezer will be disappointed in everyone who didn't immediately realize he was almost certainly telling the truth. He doesn't strike me as the type to say false things as a joke without a clearer warning that it is a joke. Unless someone can find an example of him doing that previously.