Good question. The answer is that self-causation isn’t about the entity bringing itself into existence. It’s a category of constant fundamental existence, like uncaused causation. The qualitative difference is whether behavior of some kind is an intrinsic characteristic of fundamental existence. If yes, then we’re talking about self-causation; if not, then we’re talking about uncaused causation (assuming we acknowledge the existence of causes and effects at all, of course. But if we don’t, we’re fatally handicapping our own rationales conceptually, which we can only get out of by cheating, to put it bluntly, and assuming the existence of cause/effects after all.)
A naturalistic (and possibly atheistic) version of this concept, applied to the space-time continuum, would be the reciprocative universe theory. (I don’t remember what astrophysicists technically call it; popularly it’s called the Bang-Bang-Bang Theory, where the universe reverses inflation after a point, and progressively collapses to a bang-flashpoint again, over and over forever. Last I heard, this had been ruled out as an option by various astrophysical properties, but that was several years ago.)
There are some physicists who are impressed enough with various observations seeming to indicate a real beginning to the evident system of Nature, but who want to retain their philosophical naturalism, who have tried to propose that the beginning of the universe from nothing can be explained by property behaviors (e.g. zero-point energy eruptions at the lagrange points in the vacuum between energy particles) which, however, could only be generated by (or at least are generated within) the reactions of an already-existent universe.
Without having read the exchange, I can’t tell whether this is the kind of thing that Daniel Dennett was proposing. (I suspect so, from previous experience with Dennett, but possibly he was trying to suggest a form of atheistic positive aseity instead.) WLC’s rebuttal is certainly aimed at a proposal of self-existence from non-existence, though.
However, self-existence from non-existence is not what positive aseity, or self-causation, is about. Consequently, the proposal of God being eternally self-begetting and self-begotten (or, alternately, the proposal of an atheistic IF as actually self-causing instead of existing uncaused), is immune to the kind of rebuttal being given by William Lane Craig here. (WLC might have different rebuttal attempts to launch against it, of course.)
Did that help you with the distinction? It’s a pretty sharp difference: positive aseity has nothing to do (or anyway should have nothing to do) with a shift from non-existence to existence. It’s more like the difference between a perpetual motion dynamo (positive aseity) and an absolute zero particle (privative aseity) being the eternally existent foundation of all reality.
The intuition of our infinite regressor proponent, in the text above, against what (for purposes of fictional illustration) he considers to be the only other option, is that (analogically speaking) an absolute zero particle would not be intrinsically able to cause anything (true enough); no causation = no existent effects (ditto); the zero particle is itself uncaused (not even causing itself); therefore it couldn’t exist, either; consequently the privative aseitist is no better off than the regressor is. I think that’s a pretty good intuition, but I would rather wait until later to nail it down more specifically. (Meanwhile, the regressor would still not be able to avoid a critique that he himself is proposing an IF, though, and not in fact an infinite ontological regression.)