The damage could've been done before you picked the car up, or it could've been done after. No way of knowing.
The trouble is you do not know what (if any, although unlikely) has been done at this point without stripping the engine down. As such, I do not think you can reject the vehicle based on what you have so far, or at least the dealer is not legally obliged to take it back. You might have got lucky and just have a bit of extra wear here or there meaning your engine will only last for 400,000 miles rather than 500,000 miles, or it could be fubared and dead in a week. That's why it's so important to both keep an eye on oil usage and keep communication with the dealer over the subject. Keep exact measurements and distances between oil top ups, and check it daily. If you got lucky, then no harm is done really. If not, you'll be so glad for that info before you start pushing for repairs/refunds.