Fake?
In my part of the world, a burger is a type of sandwhich, and the definition doesn't require meat. So it's a burger whether it contains beef, fish, chicken, a vegan patty, a large slice of tomato, or whatever.
This comment getting downvoted is one of the most "US Defaultism" expressions I've seen on HN. Should've posted it when the US is asleep!
What part of the world, and how recently? Sure a burger is a sandwich, likely being a spin off of Hamburg steak.
Given all sandwiches, what in your part of the world makes a sandwich a burger? I think for many of us it's a ground patty. If said patty isn't meat, yes we might say that is fake as in an imitation of the original. It's not a negative thing.