I think after exploring it a bit more, the confusion comes from the fact that:
- Despite using a multi-segment polyline in AC, the resulting wall creation through GH is multiple separate wall elements.
- In order to see and edit these walls in AC with straight wall options such as the 'offset' command, you can GROUP the segments in AC after they have been generated, and then selecting them as a group and 'suspend groups' (normal workflow).
This is related to the post
https://archicad-talk.graphisoft.com/viewtopic.php?f=43&t=59831.
Hope this helps someone who encounters the same confusion as I did.