In short, it's only at the point at which passengers are able to deplane, not at touchdown on the runway.
Irrespective of when the doors are opened, Germanwings' argument that arrival delay is calculated by the difference between the scheduled (on stand) time and the actual (touchdown) time is clearly disingenuous and has justifiably been laughed out of court.
But, IIRC, it hasn't been airline practice to record 'door-opening' times. Has it?
I believe there are specific ACARS messages for "doors closed" and "doors open" (in addition to the usual Out and In messages) but I don't think many airlines use them.