There are really two different meanings for (game) engine today. Categorical definitions are often nebulous, but here's the feeling I've picked in 30 years of orbiting the industries.
Unity already contains dozens of things we call engines. There's a Particle Engine, a Physics Engine (two of them!), an Animation Engine, an Audio Engine, and a UI Engine at the least. The object management system and rendering subsystem might also count.
In this form, "engine" means "a usually-reusable/modular system, component, framework, module, or library designed to manage the ongoing transformations of your persistent state based on inputs or rules." Things outside of games use the term sometimes too; I've seen plenty of HTML renderer and database and widget/windowing systems being referred to as engines, if not named it in the code.
One thing Unity doesn't come with is a Rules engine or a Mechanics Engine or a Gameplay Engine (unless you're making a pure physics game), and that's what that guy is trying to make. It looks like in later episodes he's working on creating mechanics for city management and stuff, so it's not just about the tile map. But a Tile Map Engine is another thing that Unity doesn't come with.
The other use of the word comes from earlier days, when intentionally making modular components to reuse between games was far less common and largely an in-house affair with things like the Z-Machine and the three or four frameworks Sierra went through over the years. It wasn't until cross-company licensing of the Doom engine broke out that anyone used the term, AFAIK. And back then, it really was one hunk of code that would run your game; you would have to hack it to add new behaviors but most of making a game like Heretic really was feeding new graphics, maps, and audio into the system.