To be clear, the incorrect Application of Law was only discovered in the changing rooms after the end of the game. The referee, on the info he had, did exactly the correct thing.
As part of the de-brief, the AR would have been given a development point of goal kicks and offsides;
AND I hope that
@Brian Hamilton did this:
That the referee got a development point on teamwork so his PMI are changed to ensure that his AR's are fully aware of offsides and goal kicks.
The lack of proper PMI & understanding the NAR's abilities led to this mistake.