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!!

How did you get the meeting? Is that a job interview? If yes, what is this job like?
It sounds to me like that you got introduced and there is no specific job on the table, so he (or she) might be a good connection to other companies as well.
The best thing in this case where there is no specific position on the table is to show were you add value. Because, let's face it, this is what business is all about. You came to clarity to get value from the experts on the site and you will get a job in the place that will recognize best the value you can bring to it. So, be yourself but also be great. Learn everything you can about this CEO (previous jobs, hobbies etc.), his incubator, the companies inside the incubator - EVERYTHING (I once learned the financial statements of a company that I was interested in... and got the job).
Show him you are interested in him, show him that you are great and make him believe that the best thing for his business will be to have you on board.
Two more things:
1. Ask a lot of questions. Intelligent ones (prepare in advance).
2. And come with a good positive attitude,not "How not to blow it" but "how do I nail it"!
Good luck!