Taḍmīn requires the borrowed portion to fit the syntax, setting and meaning of the new verse. Appending a famous hemistich to an unrelated sentence is not sufficient evidence of elegant incorporation.
Some examples let an older line complete a new image; others produce an additional allusion or tawriyah, a play between nearer and remoter meanings. The critic asks what new gain the later poem has achieved through the older words.
Thus the merit of taḍmīn depends on organic fit, not merely the presence of recognizable borrowed material.