That assumption isn't really a core part of the argument... the general "if specifying human concepts is easy, then come up with a plan for making a seed AI want to stay in a box" argument still stands, even if we don't actually want to keep arbitrary seed AIs in boxes.

For the record I am significantly less certain than most LW or SIAI singularitarians that seed AIs not explicitly coded with human values in mind will end up creating a horrible future, or at least a more horrible future than something like CEV. I do think it's worth a whole lot of continued investigation.