Putting a time limit on offers is not a big deal. I mean who wouldn't say to a good startup, that multiple parties see potential in, saying.. you're past the time limit we'll pass. Financially thats not a good idea. But in the end decisions need to be made and overall its good for all parties for yc to work with the startups that are put in this position.
At the end of the day if they originally wanted you, chances are they still want you, despite deadlines. They won't cut their nose to spite their face. Call their bluff. Chances are they'll want you back.
Not to mention the selection correlation - if you get into some seed organization, you're already more likely to get into YC. It isn't guaranteed, but it's already a good sign.
