That is God created everything in potential during the six days of creation. The everything in its proper time came into actual existence. A better way of putting this is there is a ground of existence and there are existing things. When things that are only existing in potential hit the ground of existence they become actually existing things.
Clearly this is how he understood the Ari also. And you can see a hint of this in Shalom Sharabi.
[That is the רש''ש has the sephirot conforming to an Aristotelian pattern in the future after the final correction of all the worlds. He writes about this very little so it is not well known.]
In any case this gives us an idea of what the Rambam must have meant when he said Genesis chapter one is an allegory. We can be fairly sure he did not mean the sephirot like the Arizal. I suggest he meant it like this statement of the Gra.
So when it comes to explain Genesis we now have two ways. The Gra. This would mean that 15 billion years would just be the natural unfolding of potential that was put into place during the six days of creation. The other way is the Arizal.
I should mention that the Ari seems at first glance to be neo Platonic. He builds on concepts of the pre-Soctratics and then adopts the answers of Plato to the problems they raised and then uses the Neo Platonic interpretation of Plato of Plotinus and then based on that framework he builds his vast and complete system. But the Gra held that the Ari in fact on a deeper level was going with Aristotle and the surface level was just allegorical.
I should mention that to me the Gra presents the path of Torah in an authentic way. That is in a way that is faithful to original sources. He has no interest to pervert or change the message of Torah to his own liking. And that is rare. This is why I have recommended Lithuanian yeshivas from time to time on because I see them as presenting authentic Torah.
But just in case this is in doubt--I want to make sure no one thinks I am on this path. For reasons unknown to me I have not been able to stick with the path of Torah and my efforts to get back to it have always backfired and made things even worse. I simply do not have the merit needed to be able to stick with Torah. And my life style is completely contradictory to everything the Torah says from the first to the last words. But God has at least granted to me the privilege of recognizance the greatness of Torah and also of knowing what the Torah says. So at least I know how far I am from it. But not just that--I can see when others are far from it also but make a show about how they are keeping it.
I should mention that there is no reason to assume Kant would disagree with the Ari. For example: "In the Transcendental Aesthetic, Kant is primarily concerned with “pure” [rein] intuition, or intuition absent any sensation, and often only speaks in passing of the sense perception of physical bodies (for example A20–1/B35)." [From the Internet Encyclopedia of Philosophy.] That is Kant's anschauung is not the same as sensory perception.
And I should mention also: "All of the mental faculties produce representations." [From the Internet Encyclopedia of Philosophy.] That means even what Kant calls intuition produces representations.
We see Kant could easily be with parallel the Ari. And in fact it was my experience with learning the Ari and being in Israel that convinced me that Kant was better than Hegel.