I'm sure it's possible to have oil in the crankcase that doesn't register on the stick. The difference between the high and low marks is usually about 1 quart, and there isn't much stick left below the low mark. A tiny bit of oil will make a huge splatter under the truck, so don't take that as the only measurement. The first time I changed the oil on my truck, some of the drain oil landed on the skid plate and made a splatter mess after I drove it. The plug was tight and no leak - it was the old oil that got caught on the plate on the way out.
The oil pressure warning light didn't come on? No check engine light, either? If not, it's probably not destroyed, but it's still a pretty ugly situation. Definitely don't drive it any more until you can get this resolved. Is the drain plug missing, or was it just loose under the rag? Also check the filter to make sure it's not leaking.
If the plug is in, they could have underfilled it. If it was not a dealer, they might not have known that a V6 takes 5.5 quarts or a V8 takes 6.5.
Just so we can avoid the place, which fine establishment did this to your truck?