At its core a game engine is a render loop and a game logic loop. If you're using a 3rd party renderer, your "engine code" might fit in a single source file if

At its core a game engine is a render loop and a game logic loop. If you're using a 3rd party renderer, your "engine code" might fit in a single source file if you're making a game like asteroids or tic tac toe for example. What you're talking about is a particular type of game engine: a tool with integrated editor, AAA 3D renderer, and which is general-puropose enough to support many different types of games built on top of it. That's a very very narrow way to define game engines, and imo it creates the wrong impression that games are this infinitely complex medium which it takes hundreds of people to work on at the most basic level. I think the author's definition is also too narrow, but it's perfectly valid to get into game engine development through small toy engines which are built from the ground up by one person.