You're going to get a pretty consistent response from HN because as a generalization most employees of a startup tend to join specifically BECAUSE they care. It's a biased sample pool in general.

As someone who has been on both sides of things but leans much more on the side of the fence you're asking from, my take is: they are looking for someone who drinks the Kool aid.

From a pure efficiency standpoint, caring does matter to an extent. With your side projects, you wouldn't spend your voluntary time doing something you don't care about. That excitement translates to more happiness, productivity, and (more cynically) the decent chance you will complain less if they need you to work overtime. You're in it for the MISSION so if the MISSION needs you to work until 8 tonight then you'll be gungho.

So my extremely cynical answer here is: they often care to have leverage to use on you. Is this true everywhere? Nope. But assuming it's true can often net you a sane work life balance. I would just tell you to lie. Do some research on the product or team, maybe learn what their own corporate BS is that they spout for their mission, and parrot it back to them slightly modified during the interview. When you get the job, then you can fake a little along the way but as long as you remain a high performer people will overlook your apparent lack of "excitement" because they don't want to risk losing you.

To my employer: I swear this isn't true for me. :)