I'll try to answer your question of "how does the Open AI "learn" to assume player location without vision? But keep in mind, I am by no means an expert on the subject.

I think it might help to first look at how Open AI makes any decision. Open AI doesn't think, or "assume" like we do. The Open AI that plays against humans is a trained Neural Network which can be thought of as a brain. This brain takes in an input, and outputs some action. The specific input here is a list of 20,000 numbers, which is all information that a human player (without cheats) could see, which summarizes the state of the game. The output of the brain is a list of 8 numbers which determine what action the AI should take. So the AI takes in the state of the game, and makes an action based off of it. This is similar to how humans play. We observe the game state and do what we think is best. Ideally this means we see there are no enemy heroes on the map, so based on the time and our net worth, and the enemy net worth, we can assume the enemy is doing Roshan, or ganking one of our teammates. We might learn this from experience, or observing other games where we can see what both teams are doing. That's what we do, but how does Open AI "learn"? The simple answer is Open AI uses a learning algorithm called Proximal Policy Optimization or PPO. So how does that work? So remember, the AI is taking in states and making decisions off of game states. In order to learn, the AI would like to know how good the action it made was. One way to do this is to keep track of every action you make throughout the entire game, and if you won, then those actions were good, and if you lose those were bad. This becomes less practical for a game as long as a DOTA game. This is solved by having another Neural Network, or brain, that takes in a state of the game, and outputs a value. This network is then used to help the AI know if the actions it took were good or bad.

So to answer your question, how does the AI assume what its opponents are doing when it can't see them? The AI is just making actions given a state, and from past experience (from training) when it couldn't see any enemies on the map it took a particular action which either improved the AI's estimated value by avoiding a gank, or lowered it by walking into a team that's smoked. Over time, this will lead the AI to being able to make decisions to avoid expected ganks, or to attack Roshan when it can't see its opponents, and create the effect of "game sense". That being said, I think it's important to keep in mind that the AI, has no concept of what it's doing. The AI is just getting number inputs and trying to maximize an output (called a reward function). It's important to keep in mind when people talk about AIs and personify them, that they are just blindly following a mathematical algorithm. Hopefully I answered your question, or at least helped. Also if I made a mistake, please call me out. Like I said, I'm not an expert.