Thanks for the responses so far - to answer a few questions:
- This was a connection on the same ticket - connecting flight had a VS flight number and was sold by VS, but was operated by DL.
- Connecting flight had wheels down on the tarmac 50mins before the transatlantic was due to depart. MCT for domestic to international is 40mins - but by the time it officially "arrived", we could of course have been below MCT.
- I arrived at the gate for the transatlantic 25mins before departure - gate was still open, and the last few pax were boarding.