I've been wondering the same thing. I've been checking over the past few months and yet to see the Fantasmic dining packages sell out, yet they're showing the slashes all the way through the first 2 weeks at this point. From the 1st to the 14th, the only available days are the 3rd, 4th, 6th and 9th.
I chatted Disney support, because we're there the week of the 9th and they said dining packages were not available those days. They didn't say they were full. They also didn't offer any explanation. It doesn't seem to me they are full, because at least with other reservations, that's a different icon on the date.