هُوَ وَاجِبٌ إِلَّا عَلَى أَهْلِ مَكَّةَ
Translation: It is obligatory, except for the people of Makkah.
Wajib has a distinct technical meaning in Hanafi jurisprudence. Tawaf al-Sadr is not identical in rank to the essential Tawaf al-Ziyarah, but it is not merely an optional act either.
The phrase illa ʿala ahl Makkah exempts the people of Makkah from this obligation. Specific other legal excuses are discussed later in the text; this is the exception expressly stated here.