I'm DMing the aforementioned module to my players, and something came up

spoiler don't read ahead if you don't want information about the module

The problem is getting in and out of sigil. The letter Lithara has implies that she came from sigil in the first place, since it is written in the letter that the portal leads from the market to a set of caves. I worked under the assumption that the portal works both ways, yet it seems that the book assumes the players are trapped in sigil.

Did I misinterpret something? Was she going from the caves to sigil and not the other way arround? where exactly am I being stupid?