The analysis Grice suggested has grave shortcomings, as explained in the article On Grice's Theory of Conversation: Deirdre Wilson and Dan Sperber.
There is the nonsense that
His mother is an angel.
is considered to be a violation of the maxim of truthfulness, whereas the negation of this
His mother is no angel.
is not. And if 'angel' is substituted by a synonym, 'saint' or 'good person', here, the analysis suggested by Grice does not remain the same. The relevant definitions have to be scrutinised, and someone has to decide when a metaphor becomes dead (and hence a metaphorical usage now has to be regarded as literal).
From Grice's decision to say that all figurative statements (in particular metaphor, antiphrasis, meiosis and hyperbole) violate the maxim 'Do not say what you believe to be false', it must be true that Greg Lee's answer is correct. However, a pragmatic 'Do not seek to mislead' might be a better starting point. Combining his maxim with labelling metaphor etc 'false statements' leads logically to 'Do not use metaphors ...' (or, as here, idiomatic short forms which have alternative readings in other contexts) which is ridiculous.
Gricean maxims were a good initial model, but should be realised to be inadequate as prescriptions (and they are given in the form of a set of rules).