Delighted you got a result. The better result is that you now know and understand YOUR own system. You could have remained confused and potentially stuck down the line if a little man had come in.
Given that wiring is a basic but important part of the Rachio setup, they absolutely should have been able to say that, if the common port is empty, it simply won’t work.
If they have KB articles internally, maybe they can add or amend one @drew_thayer