They didn’t send a Notice to Hirer because you jumped the gun. If you had done nothing following the Notice to Driver, they would have sent a Notice to Keeper to the registered keeper, and subsequently would have sent a Notice to Hirer to the hirer of the car.
They may conclude that you are the driver, since you replied to the Notice to Driver. Although it would seem that they haven’t; other parking companies might well do.
Anyway, APCOA don’t “do” court, so you can ignore everything until they give up, unless they change their modus operandi and issue a Letter of Claim.