The PCs are going to be in the Horn for seven months (at least!). They are going to need a lot of traps, items and magic to survive the ritual and Farholde is less than a day's travel away. They have plenty of time to use their loot or transport it someplace safer than this unholy temple of evil. The Horn even shakes, partially collapsing days before it finally collapses, giving them one last chance to get the loot out.
But, if after all that, they don't make preparations to get the loot out of the Horn before they complete the ritual...well, yeah. They might loose some treasure.
*shrug* If you really feel they've been shafted, up the amount of loot Thorn gives them at the end of the adventure. Myself, I wouldn't do it. They had months to use or put that treasure someplace safe (and the extradimensional wraith-hole in the temple of plague cult is not what I would call safe).