The issue is that they did not have access to their assigned seats in the cruise. That is the no-no. Putting pax in jump seats is fine, but if they are in the way during service, then they should have been given other jump seats if possible.
This is black and white. Passengers must have a seat available to them. If they make the decision to move about the cabin and something happens, then that is one thing. If you do not allow pax access to their seats and then you get some CAT, then the liability is on you.
I suspect the crew will be getting a grilling over this. Whilst having the wrong LOPA on the DCS is a company issue, that is an issue and to be frowned upon, but the real naughty thing is denying passengers seats in flight and therefore access to the emergency equipment required, as well as potentially putting them at risk of injury.