Here it's more a matter of knowing what was 'intended,' as opposed to what was 'written.' It can seem laughable, but the short form of the equipotential issue can be summed up with:
"Do I have a milking machine?"
That's pretty much what got the whole issue started. You didn't want your cow to step from an energized slab to a non-energized slab, or for your horse to nibble on wires without tripping the GFI.
In more technical terms, an 'equipotential plane' is one way to help 'lost' electricity get back to the panel, where it can trip the breaker. So, a key part of the issue is: where would the 'lost' electricity come from, and how would it get back to the panel?
Let's just, for the sake of argument, imagine that you had a powered conveyor bringing feed from outside, over that slab, and into that stable. In that situation, one might see the benefit to having the slab nicely bonded to the service, because a fault at the far end of the conveyor just might try to travel through the slab.
The use you propose does not require an equipotential plane.
As a design note, though, I'd suggest replacing that single light bulb with a pair of fluorescent fixtures. It'll make both the horse and the stablehand happier. You also might want to have an outside disconnect for the building (the Fire Dept. will love that!)
