Having Fire Mage as S-Tier and Frost Mage as B-Tier immediately makes me sceptical about these rankings for Mythic Plus. The Glacial Fragments legendary, which isn't even mentioned in the guide, is a must-have for Mythic Plus and makes Frost very competitive on AOE fights. It only lags behind Fire on short single target fights where Combustion's burst nuke can shine.
Noxxic's DPS rankings list Frost above Fire on 3+ targets at every gear level for damage output, and in my own sims and in-game testing I cannot get Fire to outperform Frost on 3+ targets, with Frost performing 8-10% higher