فإن قصد تشبيهها ... فهو استعارة
If the usage intends an analogy, it is metaphor.
The source considers a term such as *mishfar*, conventionally associated with a camel's lip, applied to the lip of a human. If resemblance of the organs motivates the usage, metaphor is possible. A slip of the tongue or an accidental misnaming is not a rhetorical metaphor.
The decisive question is not merely which object the hearer eventually identifies. It is why the speaker transferred the term: likeness, another admissible association, or no figurative intention at all. This clarifies why an apparent substitution must be examined in context.