If the driver had stopped to load or unload, it is not considered parking. At least according to some persuasive case law. Although this is on private land and the scammers can make up the rules, there is no prohibition on stopping on double yellow lines to pick-up, drop-off or load and unload.
However, the keeper is now dealing with a vexatious firm of nasty ex-clamper thugs. It is all going to depend on how badly the Keeper wants to fight it.
As the PCN is an invoice for an alleged breach of contract by the driver, you have to ask, what contract? A contract to be valid, requires three elements. 1. An offer, 2. An acceptance and 3, consideration. A prohibition does not offer anything. Therefore, there can not have been any contract in the first place.
As the Keeper will be dealing with an IPC member, there is only one way this is going to go and the two likely outcomes are going to lead to a debt claim in the small claims track of the county court. The most probable of those two outcomes is that they will eventually discontinue before any hearing takes place.
The second possibility is that it does go all the way to a hearing where there is a greater than 50% chance that NPC will get spanked in court.
I’ll leave it to others to explain the obvious downside of being incredibly unlucky and losing the fight. The only other option I can think of is for the keeper to pay it but, personally, that goes against the grain, especially if the keeper feels aggrieved about this PCN and that it has been issued unfairly and incorrectly.
These operators rely on the fact that most of their victims are low-hanging fruit on the gullible tree and will fold at the first hint of litigation, out of ignorance and fear. We assist the victims fight these vermin every day.