Say you have linked against a GPL'd dependency which you have not modified but for which there is no API-compatible substitute. Are you now bound by the terms of the GPL? FSF says "yes", which tends to surprise people. But if there is an API-compatible, non-GPL'd substitute, the answer arguably becomes "no". Note that this has nothing to do either with your work or with your direct dependency, but on the existence of some random artifact out in the ecosystem which may come into existence at any point in the timeline.
API is not a random artifact but a dedicated feature designed to separate one program from another. It also implies much more stability for the external connections to the code.
It's like saying that display is a "random artifact" of a computer not making any difference in its design or usage.
