Because it's still insanely impressive. I would have been blown away if the AI was merely at the platinum level. This is an AI playing starcraft. Starcraft!
The AI normally picks a set strategy and attempts that. On harder difficulty the AI gets to cheat (more income per mineral collected). The SC2 AI is extremely primitive and at best Bronze level. The AI never really "reacts" to your actions, it's basically always on a set path. DeepMind is orders of magnitude better than the Starcraft AI.
FWIW cheating AI in RTS games is extremely common. Often they don't get fog of war, or have higher income rates. RTS AI is very very hard, so this is very impressive.
