I suspect this is really a corruption of the principle that nothing a liar says can be trusted.
To label someone a liar is to say that their utterances are known to be untrustworthy and their intention known to be manipulative - that it will never be apparent to the listener whether the liar is telling the truth or is practising to deceive.
The status of being a liar is accrued by their being merely inconsistent in telling the truth and using information to manipulate, not by their every word being false.
It is rarely the case that a liar always declares false information, such that the truth could be inferred by merely interpreting the opposite of the information conveyed by the liar.
From such a ridiculous status point, the liar could always increase the manipulation, and break the ability to infer the truth, by occasionally telling the truth.