Well, year 2000 graphics in an isometric game meant most NPCs were so heavily pixelated that unless they were outright naked or had grotesquely over-proportioned body parts (Andariel, Succubi in general, etc…) that gender was not necessarily a given by appearance alone. ‘Hratli,’ doesn’t necessarily come off as a masculine name compared to the names of other male NPCs, and the difference between robes and dresses can also be difficult to make out with early 2000 graphics. Short hair wasn’t necessarily a gender-distinguishing feature either, considering the Assassin’s hair is short.
All in all, I’m not completely shocked that there are people who thought Hratli was a woman if they never paid attention to NPC chatter. I never thought he was a woman myself, but I’ve always been the kind of nerd who loves story and listened to every NPC on each step of every quest.