It’s one of the chosen few that don’t need to be encased in pastry, yet still be a pie. Fish and Cottage being the other 2, assuming a mash topping
There’s a logic to this, which I covered on an earlier thread.
http://singletrackworld.com/forum/topic/oi-binners-what-have-you-started/page/4#post-7565463
I think we can make an exception to names of pies that aren’t defined by their ingredients. That is to say, a pork pie is a pie with pork in it. A shepherd’s pie does not contain shepherds, and similarly a cottage pie generally does not contain cottages.
Ie, “shepherd’s pie” is the full name of a dish that is not actually a pie, in the same way that an Eccles cake is not actually a cake. See also, beef wellington.