“Game engine” is supposed to simply mean, the parts of the game code that aren't the main game logic—the stuff that handles asset loading, entity management, input detection, rendering, sound playback—that sort of thing. It could be used to refer to Unreal Engine… or it could just mean the simple, minimal homemade tech stack that you built your game upon. Or anything in-between.

But you hit the nail on the head—it's definitely one of those terms that has been warped and downright mythologized in recent years, with the rise of free-to-download general-purpose GUI-editor “game engines”, and their ensuing popularity.