>>63830052
According to Halo's lore an orbital MAC gun can fire a projectile incredibly fast, so it might be able to take down a star destroyers energy shields. They can one shot covenant capital ships with their shields at full power.

https://www.halopedia.org/Er%C5%91d-class_orbital_weapon_platform

We saw what a ship going light speed could do to a star destroyer in one of the movies, so just shoot it 3 or 4 times?