If they crash it, yep, TPK.
Of course, "crash it" in this means "The Castle fell straight down from its cruising height." Said cruising height is well over 200 feet up (apparently, the average cloud deck sits ~2000 feet above ground level). Of course, that's not the scenario the adventure gives you. It's talking about crashing the castle into a mountain/glacier.
There are problems with that:
1 - the Well of Dragons (where all that treasure is going) is to the south, directly opposite the site the adventure assumes.
2 - The castle's speed isn't given. Assuming a Giant's walking pace, the best estimate is about 6 days to the presumed "crash site".
The end result: this scenario is probably the weakest bit of HotDQ. Rewrite it as it appeals to you.