Seems that the wiki needs updating to match the current Arch Building Part (BP) code. From my end when a new BP is added, on the Properties View tab, under Interactions, both the Auto Working Plane and Double Click Activates are set to False whilst Set Working Plane is True.
Setting Double Click Activates to True allows me to double click on the newly created BP and once activated (it shows a blue background), even though I may be highlighting somewhere else, adding another object like the Wall does indeed add to the now active Building Part as you queried.
I assume the working plane functions as required (dependent on the noted above settings on the View tab) although I didn't test that aspect.
Once you confirm the operation, can you please update the Wiki? Anyone can have a Wiki account to update the online documentation as you require. As vocx said in one of my posts on similar matters, the Wiki pages often lag behind the fast-evolving code and updating by users like yourself and me is highly encouraged. I've already recently fixed a few minor Wiki entries.
Hope this helps.
yesterdays freecad-daily PPA
OS: Linux Mint 20 (X-Cinnamon/cinnamon)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Build type: Release
Python version: 3.8.5
Qt version: 5.12.8
Coin version: 4.0.0
OCC version: 7.3.0
Locale: English/New Zealand (en_NZ)