إِذَا قَوِيَ الشَّبَهُ بَيْنَ الطَّرَفَيْنِ حَتَّى اتَّحَدَا
When resemblance between the two sides becomes so strong that they seem united.
This does not assert actual identity. The association can become so strong in expression that repeatedly restating the comparison seems unnecessary. Metaphor can then express the recognized relation with greater compression and a stronger rhetorical claim.
The unity is restricted to the specific intended aspect. The objects remain distinct in reality even when the speaker presents them as nearly indistinguishable in that respect.