I found some similarities with what occurred with Deepmind's Alphastar AI.
One of the weaknesses that seem to manifest in this piece too is the handling of unfamiliar scenarios.
The AI is very confused once it experiences something that was rarely seen in its learning data. Destroyer's big drones confused the bot quite a bit.
Deepmind solved it by intentionally creating agents that introduce different\bizzare strategies(which they called exploiters) in order to develop robustness against such strategies.
Completely agree that adding something like the "League" used by AlphaStar would be one of the top priorities if you wanted to push this project further. I don't think CodeCraft is sufficiently complex to really allow for several very distinct strategies in the same way as StarCraft II, but I would still expect training against a larger pool of more diverse agents to increase robustness quite a bit.