"Metro style service" means that, for the majority of the time trains are running there will be a turn-up-and-go service (in practice this tends to mean 4-5tph or greater), it doesn't imply anything about the start and end of service.
Every railway line needs to have a period without traffic so that it can inspected and have maintenance done on it. Given the very intensive use by both passenger and freight services, there will probably be very little opportunity to do this on the NLL so Sunday mornings are probably the least worst time possible.
The essential things in life are seen not with the eyes, but with the heart. --Antoine de St. Exupery
When I worked at Stratford we had to open up at 6am because there was a 6:15 into Liverpool Street while all other Tube stations around us didn't open until 7am.
We used to have the clubbers from the Rex waiting outside the Boswicks for us to open up, when we did open they'd storm through/over the gates cos loads of them didn't have tickets and rush off to the platforms.
About ten minutes later a few of them would drift back to the gateline to ask when the first train was. 7am for the Central and Jubilee lines (westbound). 7:30 Central line (eastbound). The looks on their faces was priceless.
When we told them about the trains into Liverpool Street (6:15 & 6:45) they'd hopefully ask if they could get the Tube earler at Liverpool Street. Er,no.
The look on the faces of those waiting for the NLL was even better because both platforms had trains out-stabled overnight and they didn't start up until 9am.