Simile states a resemblance between two terms, often independently existing. Tajrīd begins with one bearer and imagines another comparable bearer emerging from it.
“Zayd is brave like a lion” contains two independent terms and bravery as the point of resemblance. In “la-tasʾalanna bihi al-baḥr,” the sea of generosity is imagined as extracted from the actual man's abundance.
The figures can share a resemblance relation, but not every simile is tajrīd: extraction from the original is the additional operation.