Godot is a game engine.

SDL, on the other hand, is not, and instead is a multimedia layer (middleware) often used for game development.

One could argue that game engines constitute as middleware, but in reality, most modern game engines are way more than that, and instead often rely on other middleware nowadays (e.g. OpenGL, or even SDL for some). This, alongside with people mistakingly calling SDL a game engine, leads to stuff like this.