Not every phrase inserted inside a sentence is a parenthesis. An adjective, circumstantial accusative, or verbal complement may be part of its normal syntax. An i‘tirad is a separate clause serving an additional purpose between connected elements.
«جَاءَ زَيْدٌ مُسْرِعًا» — «مسرعًا» is a circumstantial accusative, not a parenthesis.
«جَاءَ زَيْدٌ — رَحِمَهُ اللَّهُ — مُسْرِعًا» — «رحمه الله» is an inserted prayer.
The first reports how Zayd came; the second adds a distinct expression of concern while keeping that information intact.