The AI "cheated".
The stated goal of deepmind was to create an AI capable of beating humans via strategy alone, by limiting reaction time and APM. Everyone already knows that a perfect micro "dumb" bot can wipe out any human player through mechanical perfection, so they were trying to test how "strategic" an AI can be by limiting to human level mechanical ability.
But the AI had vastly superior APM/EAPM bursts that were physically impossible by humans. It's unclear how they allowed that to happen, but it looks like an oversight.