To be fair it says right on the AVGN store page under System Requirements that it requires Windows. The Steam Store is a website that anyone with a browser can access, regardless of what OS they run. I wouldn't expect Newegg or Amazon, for example, to only let me buy computer hardware that is compatible with the OS I am currently browsing from.
Though if you did it through the Steam client itself I can see the confusion... it should probably warn you at checkout in that case that you would need to own a different PC in order to play your order.
Currently I think the only warning is shown when you launch the game, and your hardware requirements don't seem to match up with the game's. In the cast of most hardware requirements the Steam Store can't tell while you're web browsing, but it can see your OS so they could check it earlier for that factor.
In the future you can use this page to browse Mac games on Steam.