It's meant to operate purely on data viewable on the screen to mimic a human player's experience and provide an even playing ground.
From what I'm reading in the paper, it sounds like there is some custom interface in play:
>AlphaStar can target locations more accurately than humans outside the camera, although less accurately within it because target locations (selected on a 256x256 grid) are treated the same inside and outside the camera.
It's really hard to parse what such statements mean. The fact that someone who is cited as a co-author of the paper approved the interface as "fair" isn't particularly reassuring.
They are not just some random person, they were top SCII players. Who else if not them would know this well enough to make assumptions?
There is a custom interface in the sense that the bot does not read pixels from the screen - it reads the information through the API, same information that is usually presented on a screen. But the amount of information is limited to exactly what a human would see at the same time using the standard SCII interface.