That is definitely paint damage due to chafing - no doubt about it. There is no metal damage which would suggest an unknown 3rd party's intervention.
Plus, their assertion that it would've happened very soon after the car started being used is quite correct - fact of the matter is that YOU were completely unaware of it, and THEY are the one's who have told YOU about it. So no-one could say when this happened (or originated).
I'm pretty certain that if you had noticed this, it wouldn't have been left to the first service, which if you think about it, gives even more gravitas to the 'there from production line' stance - if it had only appeared by the second service, this would be a bit more uphill to claim.
I would say that either the light units have worked themselves loose (and are thus vibrating whilst the car is moving), or that the tailgate and lights foul causing the lights to move.
Either way - it's not you, and it's not some miscreant creating mischief.
rather than delay matters further.
Best of luck, and keep us updated.