Shutter speed has nothing to do with the flash duration. You can have a shutter speed of 1/10th sec (to get background light) and the flash that's 1/10,000th sec to light the wolf.
Add-on: If the light source was constant (not strobes), then I think the shutter speed argument stands. However, I can't imagine that this wasn't done with strobes. Furthermore, that the background was exposed at all suggests some shutter drag?
I am realizing that I need to spend more time playing with my cheap off-camera flash set-up.