Pot pie isn’t really pie at all, it’s stew. “Potpie” was just the name stew went by in some quarters of the English-speaking world, until somebody somewhere (possibly in Pennsylvania) decided to take the term literally and plop a top crust on it. That most likely occurred sometime in the 20th Century. Prior to that point “potpie” was nothing more than meat and vegetables cooked up in a pot. Frequently chicken or rabbit, less commonly beef.
If that sounds odd, consider that for most of the history of pie, a pie crust was not the object but the means. It was the stuff inside the pie that counted. The crust was just the thing that held the contents together as they cooked. And an awful thing it usually was too. Nothing more than a paste of cheap, coarse flour and water, sometimes an inch or more thick, that was frequently blackened by the time it emerged from the oven. Most people threw it away once the pie’s contents were consumed. Rich and flaky edible crusts are a much more recent phenomenon.
Seen in that light, it makes a bit more sense to call stew a “pot” pie, no? If the crust is just the vessel, then what difference does it make if you swap it out for some other vessel, maybe one that can be reused endlessly — like an iron pot? Works for me.