Good micro has a lot of room for tactics and outplaying where there's a lot more to it than having a high apm, so it's not really a surprise that Deep Mind can outplay the AI, even with a lower APM.
Evenly matched armies looking for a micro-based edge turns basically into a fighting game like Street Fighter, where it's about reading your opponent, baiting them into a mistake, pre-empting them, taking decisive action, crisp reflexes/timing, doing something unexpected or unorthodox to get an advantage at a huge risk, etc.
