أَنْ يَكُونَ بَيْنَ الْجُمْلَتَيْنِ تَبَايُنٌ تَامٌّ بِأَنْ يَخْتَلِفَا خَبَرًا وَإِنْشَاءً
The clauses are fully different in function: one declarative, the other nondeclarative.
For instance, one clause informs while the next suddenly commands, prays, or asks something that is not meant to share the first clause’s construction. Separation preserves their different purposes.
The cited poetic passage contrasts the directive arsu (“halt/anchor”) with the declarative fa-hatfu kulli imri’in yajri bi-miqdar (“each person's death proceeds by decree”). This difference motivates fasl.