It's probably just some college kid playing a prank.
A complicated one because ABC picked this video up in Afghanistan, guys it may look cheesy but it is a threat to us nevertheless and needs to be taken seriously, remember this guy probably lived in the US and he knows the popular culture and probably knows or has a pretty good idea of initial reactions to it. There no standard for terrorist warning videos, they may come in many forms.
You know some college kids that could hookup an As-Sahab logo (do you even know what that is?), Arabic subtitles, and Al Qaeda rhetoric so well that the ISI could vet it, and then manage to get the tape transported to Pakistan?