Right, but if Thanissaro believed that akuppa meant that release is not disturbable by greed, hatred and delusion, he would've stuck with 'unshakable.' But he changed the meaning of the utterance from 'The Buddha's liberation was fixed' to 'The Buddha's liberation was uncaused.' The commentaries, too, seem to interpret liberation as 'unshakable.'
"What holds attention determines action." - William James