The 2 hour refund window is for automated refunds, you can still make a request if you're past that - it's just going to need a human to take a look at it. I've once succesfully returned a game I've played for about 5 hours because it had game-breaking bugs and ran like crap for no reason, and it got accepted within a day without an issue.

So Helldivers owners have a chance. I'm assuming that Steam's Customer Support department is having some kind of an internal discussion right now on how to handle this case.