That isn’t DM mode, it is a very nice set of customization optimizations.

In Neverwinter Nights there was a DM mode that let one player be the DM instead of one of the characters. They could drop in creatures and objects and control them in real time, which was awesome! That is what I was hoping they would be adding, something where we could basically play DnD in the world maps of BG3, but with on the fly encounters and pacing instead of being preset.