Yes, it's all rather disappointing.
First of all Shadowcurse, outside one scripted story events, hardly makes an impression - and very quickly it stops being a thing alltogether. On top of that mountin pass enterace and underdark enterance are in the same map area - maybe there is an extra scripted "uuu, you chose an easy but dangerous path!" if you follow mountinpass, but it does seem like Halsin was just talking out of his ass. Ah yes, you should pass through underdark and an enemy fortress to enter shadow curse a bit to the east.
It seems to me like they were setting a much bigger choice that they turned away from. BG3 gives me this weird feeling like they were making it as they go along - building content from the beginning to the end. Which I don't think is how usually the games (or stories in general) are made.