Sorry, but that is wrong, the hydrant is that long so the valve can be below the frost line, otherwise it would freeze in the winter. The length depends on how deep the frost line is in your area. Here are some for https://www.muellercompany.com/water-works/hydrants/fire-hydrants
It looks like you're right for at least a couple of those examples. That's cool, I learned something today. So what's up with the bolts at the base? I guess the valve part slides into the main pipe so they can easily replace it when needed?
I'd imagine it also makes repairs easier. Say a car hits a fire hydrant. It's really only going to damage the part above ground. Having that part bolted on means you don't have to dig out and replace the entire thing.