Sajʿ produces correspondence between endings of separate prose units. Tashrīʿ is a single poetic verse intentionally admitting two different valid rhyme stops.
Sajʿ is assessed through prose endings and their sounds; tashrīʿ must additionally satisfy poetic metre. A sequence of similarly rhymed prose clauses is therefore not automatically tashrīʿ.
In “yā khāṭib al-dunyā...,” the relevant feature is not two separate prose units but the two possible completions of one verse.