You're correct in that you only have to visit the area and not get 100% (because that would be a bit ridiculous for stuff like The Veil). I got it after visiting the developer fish room (I had already gotten the full ending as well, but I don't know if it counts that since you can't save after doing this).
If you don't have everything, I suggest getting whatever you're missing, such as treasures or the third cooking slot from the simon says boss.
However, you said you'd checked the treasure hunter map, which means you may very well have everything. If that's the case, it may be an achievement bug that needs to be worked out.