Your original characterization of the so-called '4th' wire was ambiguous. Not even ambiguous, more like (unintenionally) misleading.
Effective communication is a responsibility primarily of/on the sender, only secondarily the receiver.
The sender is supposed to recognize the limitations of the media and know that the receiver cannot see inside the head of the sender to know what he was thinking about when he said what he said.
You said at the opening:
3 like wires and then 'the other' would cause a normal reader to call the other the 4th, different, wire.Then you said:
I believe that it would be normal for the receiver to interpret that the bare, neutral, different, 4th wire is not connected.
Your original does not 'select' one of the 3 otherwise identical black insulated wires to be the '4th' wire (not connected).
The criticism of the communication breakdown should not be assigned to the receivers comprehension or lack or mind-reading skills here.
It wasn't until later that you began to try to clarify what you were thinking and seeing at the wires.