وَكَمَا أَنَّهُ مُحْيِي الْمَوْتَى بَعْدَمَا أَحْيَا اسْتَحَقَّ هَذَا الِاسْمَ قَبْلَ إِحْيَائِهِمْ
Just as Allah is entitled to the name “Giver of life to the dead” after reviving them, He was entitled to this name before their revival.
Muḥyī al-mawtā means the One who brings the dead to life. The name does not denote a perfection first acquired on the Day of Resurrection.
The event of revival occurs at its appointed time; Allah was already entitled to the divine name.