"This universe" is the first universe.
What the one and only true "Big Bang" is used for,
to create "This Universe" which leads to the creation of all others.
Because 616 is the first universe,
and every other mirror/diverged/parallel universe
is only relevant/existing because 616 was first.
They're all copies of one template, the first template, 616.
So, if one goes back to the point before 616 was created
in the original FIRST Big Bang that spawned the Multiverse ...
... they end up in the absolute void of nothingness where nothing else exists anywhere else.
In Siseneg's case he absorbed all the energy across 616's history,
616's history is intrinsically tied to the history of all the alternate realities.
--------------------------------------------------------
This is why the writer of even that scene you posted
that does not involve the creators of the original Siseneg story,
still labels Genesis "GOD!"
--------------------------------------------------------
Now, exactly how the writer in the LT involve scene interprets "GOD" is unknown to me.
But what I do know is an undebatable fact,
is that according to the Writer and Artist of the original Siseneg story,
he became "GOD, the One, the Almighty - Ruler of All RealitieS" ...
... hell, he even created the Writer and the Artist on panel.
but who's counting? 
