I think 'orbital' is more than fair to describe the longer term aim of the london overground. However, I think it is pretty pointless joining up local services to do this. A London orbital should be a fast service connecting ~5-10 major transport hubs in outer london - East Croydon, Lewisham(?), Heathrow, Watford, Romford(?), stratford(?) etc.
I should be able to get to the other sides of london in 30mins. After all, I can get to Brighton in under 1 hour.

Overground Orbit:
Crystal Palace to Clapham will be the only section of the orbit that is not the Overground, and that may change if we lose the London Bridge/Victoria service.

The west london line (Clapham Jumction to Willsden) is already part of the overground.
The north london section of the Overground connects Willsden to Dalston.
The newish east london section of the overground will (eventually) connect Dalston to Crystal Palace to complete the orbit. (via a Dalston Junction?).

The underground map always had spurious rail links on it.
The tube is a very frequent (2/3 mins) service that goes through central London. Most people in se23 need to journey into London too. A tube service would be ideal. This orbital service aint it.
This is why we must not lose our existing connectivity to London Bridge to accommodate this new service - and that is something I think we can all agree on.