"I know I had a right hand drill bit, but I don't remember if I was drilling from the front or back of the hub. Anyway, the screw spun right out easily. It was actually loose. "

Yes, if you use a right hand bit, it will push it on through. Left hand removes it from teh front side. If there is no issue with the screw coming out the back and getting hung up, right hand will also work.

" Anyway, the screw spun right out easily. It was actually loose. So the question is then, how do you break off a screw that is not seized? My guess is the mechanic had his impact set the wrong direction and snapped it off."

I often end up removing screws and bolts that have been broken off both on assembly by overzealous impact users and also on disassembly, when the shank of the screw is rusted or corroded in the clear drilled part being held by the screw. In both cases, the screw can usually be removed with your fingers if it were not broken off below the surface. I have also removed some by using a sharp pick and simply unscrewing them until it can be held with your fingers. Only gotcha is when the screw wrings off and buggers up the first couple threads or the threads themselves are corroded stuck. In that case, sometimes you can TIG weld a nut to the screw through the center and remove with a wrench or it may just be hopeless to remove short of drilling out and re-tapping the holes. Worst case, you helicoil it, weld up and re-drill and tap the hole, or replace the part, in that order.