For those of you who think there won't be any walk up availability, I would like to share my experience.
I checked into the YC on 12/24 (yes, Christmas Eve). I'd been checking from the evening before and all day to see if there were any openings at B&C, but it was showing booked until after 7pm. I arrived at the hotel about 3:30pm, still checking, still nothing available. So I decided I would go in person to check and see if I could maybe get in, and if not, I'd just get an ice cream to go and head to Epcot. When I got to the hostess stand and asked if they had any openings, imagine my surprise when the place was literally empty! There was one table with a family sitting there, and no one at the counter at all. In fact, they were talking the whole time I was there about how no one was coming in. It was maybe a third to a half full when I left.
I don't know where the disconnect was between the app, the website, and the restaurant was, but I was glad I decided to check. I can't guarantee your experience will be the same, but if you're dining at an off time, it's certainly worth checking on walk in availability if you can't get an ADR.