The information is only imperfect for a human but an IA can have a way better access to information: - if you scout you opponent mineral line and click on the minerals, you know exactly how much minerals he's harvested since the beginning of the game. If you scout your opponent army and building, you know exactly how much he spent. Then you can deduce if he has something hidden somewhere or in production. Of course this is impossible to do for a human, but really easy for a computer. - scouting is easy but requires a lot of actions, an IA could have one mutalisk wandering over the map the whole game without being killed, which is impossible for a human unless you dedicate full time on this.

Imho, beating humans to StarCraft isn't a big deal in itself, but it would need a lot of work to build the AI to deal with the game's complexity. But it's merely domain specific work that has little to no interest on AI in general. Creating an AI that trains itself how to be good at StarCraft will take much more times and effort, but at least it will provide value outside of the specific problem.