May 4, 2016 - 3:01 PM
I wouldn't worry about it. Tickets rarely sell out (see here). It's indeed just the seat reservations that may have sold out, but even that does not mean there won't be seats left as they won't sell all the seats online. This may even be a technical error as their site showed an error message, not a booked out message, when I tried your date.
The general approach to such trips is not to book in advance as the weather makes or breaks the experience. Only tour groups with a fixed itinerary should book in advance, but if you're flexible you can just wait until the day before or the same morning, watch the weather forecast, and decide whether you want to go up or not. You can buy your ticket the prior evening if you want to be on the safe side.