How is Protagoras' relativism self-refuting?

Validity of Plato's argument against Protagoras' relativism at Theaetetus 171a-c has been called into question by many scholars, largely because the argument, notoriously, omits the qualifying phrase "for x", the phrase of vital importance to relativism. Recently Luca Castagnoli offered a valuable interpretation of the passage, according to which Plato's argument does not aim at proving the absolute falsehood of relativism, but successfully shows that it turns out to be untenable for Protagoras himself in dialectical contexts...