A bird in hand is worth two in the bush. But, you're trying to hold the entire bush, keeping birds inside. You want a job, want to work inside the incubator, but don't want to work for startup.

I'd advise you to listen to the CEO and try to understand his perspective and preference. But, don't accept or commit to anything as long as you aren't one hundred percent satisfied inside to accept the same. A polite "No" in the beginning is better than numerous alibis that may happen down the line. Listen and listen more to understand, then speak. All the best!!