I cannot bet on it but it is very likely that the trees are lighted up either with the low sun or with the clouds diffusing the light from sun shining behind the horizon (you can see some clouds on the photograph) - the shadows areund tree details are made by something shining from horizon.
The scene is additionally backlit by the moon - the shadows from trees are soft.
The photograph might be composed from two separate exposures — the sky and the foreground — and merged accordingly but it is not clear whether it is required or not. If there were two exposures they could be made with big interval - one before the sun went down and one after the twilight. If there are specific NatGeo's policies preventing the compositing the photographer might have used the camera's double exposure function which was introduced recently.
I think that the lighting is not artificial: neither the electric lights (the sodium lighting kills almost all chromaticity even if the photograph is white balanced but the colours of trees are definitely distinguishable) nor the flash (the light is too warm and does not seem to be white-balanced in post-processingб there are also some deep shadows around details of trees which photographer could have avoided if they used artificial lighting).
TL;DR: you can make amazing photographs without strobes. :)