Zawāl is a solar time marker, associated with the start of the Dhuhr period; it is not synonymous with attending the Dhuhr congregation.
If a pilgrim throws the pebbles at 9 a.m. on 11 Dhu al-Hijjah, the fact that daylight has begun does not fulfill the source's after-zawāl timing. The precise condition is the sun passing the meridian.