Cap'n Kobold
Hero
I mostly meant in terms of its location and situation. It is pretty much the sole effective entrance to the island. Much of the traffic utilising it are adventurers, and elite military or special troops: not only a much higher proportion of 'PC-classed' customers, but also of generally higher level than most other settlements would see. - The type of people most likely to buy magic items rather than artwork or trade goods.It's not really much of an exception, since it fits directly in with the rules on page 135 of the DMG. It sells a few minor things, but rarer items aren't for sale. I guess it's somewhat of an exception in that it specifies it where the other modules don't, but since the rules specify it, those other modules can have places like that, too if the DM wants.
Most of these adventurer types are travelling into the jungle, towards the ruins. These are both highly hostile environments with many supernatural threats as well as the native dangers. - The sort of situation where buying magic is actually worthwhile compared to simply hiring 20 more mercenaries.
If anywhere is going to cater to such an adventurer-specific economy as selling magic items, it will be that city.