In the case of a Logger, though, it is likely passed around by reference or pointer. So users of the Logger (reference/pointer) do not have to change if its definition changes to that of an interface.

The "Server" class, or whatever instantiates the Logger, will have to change, but that might not be such a big deal.

So yes, the author was wrong about not having to modify any code, but likely you won't have to modify most code.