Maik, assign-wise your "please note" is correct, but action-chain-wise I think it is not.
To deal non-combat damage you have to take an action.
With that action you started an action-chain.
In the action-chain I use the Catapults Action.
As it is last in, it comes first out, so I sacfifice the unit targeted by your non-combat damage dealing action and deal 2 dmgs to capital.
Then your action would come, but its target became illegal, so your action is just gone for good.
Sure, if I do not respond in the action-chain, then the Catapult cannot be used after assigning - before applying.
All in all, different mechanism with same result: sacrifice instead of destroying.