Another way to think about it is: you don't need insider knowledge, you can get an advantage in all kinds of ways. For instance, there are innumerable biases in how people think about stuff -- you can find lists of them all over the place. If you can manage to be less biased than other market participants, that's an advantage. You have access to the same info as everyone, you just use it better because you're less biased.

If you can bring information to bear that the market doesn't include, that's an advantage regardless of whether or not it's 'insider' knowledge. It's often the case that key info is sitting out there in plain sight, but for one reason or other -- often due to the aforementioned biases -- people don't include it in their worldviews. That's alpha for you.