The connection from my gas meter to pool is both. It's starts out a solid metal pipe, transitions to a plastic pipe before disappearing underground. It reappears at the pool equipment pad where it turns back into a metal pipe.
I think what they are worried about is the line cracking underground, hence the use of a flexible plastic pipe. It also won't rust so being exposed to the damp/moist soil isn't an issue.
At the pad, I'm sure a sufficient frost heave could break the metal pipe fittings but there it's all exposed. It should be easy tell if something were wrong. Underground, not so much.
The important thing is to have a gas shutoff on the pool line at the meter. Close the valve during winterization. When you open the pool in the spring, turn it back on. Just make sure you check the lines at the pad for any obvious signs of a leak before trying to use the heater.