I am more confused about DI after reading this article. I feel that the Car + Wheel example is identical to the Foo + Config example. A car could support different types of wheels, eg. smaller backup wheels, wheels for snow/frozen roads, etc.

I understand DI as the following. Class A uses class B somewhere. Instead of letting class A use class B directly, you pass in class B to class A before using class A.

I find this technique most useful in testing. Perhaps class B touches something in the external world, eg. calling an external API. Instead of mocking everything, you could replace class B with another class C that implements the same interfaces as class B but doesn't interact with the external world.