Article author here. Your idea "gluing together things that don't know necessarily know about each other" is basically what the GoF book means: composition is "this object has a reference to that object and uses its public API". They don't mean "this object ontologically contains an instance of that object" in the sense that a car "has" an engine, which is a narrower definition of composition that people frequently use.
It's that broader version of composition—particularly in its extreme realization, delegation—that underlies a lot of the behavioral patterns in the book. For example, the State and Strategy patterns boil down to "this object relies on another object to fill in the behavior here, and there are ways to choose what that other object is", which is something it's easy to arrange with subclassing and the only point of the pattern is to avoid subclassing.
It's that broader version of composition—particularly in its extreme realization, delegation—that underlies a lot of the behavioral patterns in the book. For example, the State and Strategy patterns boil down to "this object relies on another object to fill in the behavior here, and there are ways to choose what that other object is", which is something it's easy to arrange with subclassing and the only point of the pattern is to avoid subclassing.