It doesn't matter if he could escape if he wanted to, because he couldn't want to escape unless he already had done so. Friendliness is stable under self-modification.

Edit: I managed to type that whole thing without quite realizing how perfect Parson is as a lay-accessible model for what an alien intelligence looks like. The perversity of his ingenuity, stemming from the fact that he doesn't share the prejudices of the people around him, is a major part of what people fail to anticipate in AI.