Rhetorical metaphor makes a claim of identification for expressive effect. Its indicator tells the listener that literal membership in the source class is not intended. Consequently, it should not be evaluated as if it were an ordinary false factual report.
“He is a lion” can praise a brave person without claiming biological identity. But a deliberately false claim with no recognized figurative context does not become metaphor merely because it is exaggerated.
Distinguish the cases through context, intended meaning, and resemblance.