Robin Hanson isn't the one saying it's wrong. A number of other physicists (e.g. Garrett Lisi as mentioned in the post) have said it's wrong, on the basis that the calculation is easy and when they do it they get a very different result. Hanson is just the one who claims to have found the particular error that the paper made, because nobody else thought it was worth bothering to find the particular error in a paper that got such an obviously wrong answer.