They didn't come up with the format by themselves. This is the "standard" 1v1 format. But yes as they acknowledged getting the bot to play 5v5 is a much bigger challenge.
Building a cooperative bot for 5v5 will impress me a lot more. People are rather good at this sort of thing and so far AI is more likely be become good at 1v1 games (Go, Chess, etc). If there is anything I've learned after 3 decades in this industry is that people are devious.