In his video Heiser seeks to lampoon the Genesis account.
"the sun created after days...how does that work?"
Here's how it works. 'Day' is a unit of time; it is the unit of time that circumscribes our experience of life within the creation. As a unit of time, it needs to be calibrated. Thus, what type of 'day' is this? Here's the calibration that tells us what type of day: it's an 'evening and morning' type of day. This makes it the type of day that we know, experience and live within. A unit of time passes without any marker necessary (so we don't need to wear a watch to know that our lunch hour will pass in one hour); indeed that's why the markers of time could be created after the unit was communicatively established to tell us that God created in the history that we experience.
The creation week was not merely contiguous with our history, but it was continuous with it. God shows that the creation is not a pagan myth off in fantasy land, and God is not the pagan nonsense god (compare the trash-talk in Enuma elish), he is the one active, present and personal in the historical domain that we are in. The 'days' serve to place the creation in the continuity of the reality that defines our life-world. God is thus the God near, present, and with us (Genesis 3:8 illuminates this), setting the stage for the theophanies throughout the Bible and culminating in the Incarnation!
Thus we don't need the speculation of some who imagine a 'temporary' light source to produce the diurnal lighting variation of the the day, all we need is the names of times in the day to denote the type of day. The irrelevance of lighting variations is evident if one has traveled in the arctic circle in summer. At 11pm one knows it is late evening and time to sleep, even though there is sufficient light to read a book by.
A related issue often used to misunderstand the Creation is the creation of light before the sun. Francis Schaeffer deals with this quite adroitly in Genesis in Space and Time (although he fluffs on the historical significance of the 'days', inadvertently de-historicising the whole creation) in pointing out that the creation of light indicates, must indicate, the creation of the entire energy field that is basic to the physical cosmos. 'Light' had to be created first for the rest to follow, something the ancients could not have known. Without its energy substrate, the sun would have been a mass of inert de-energized 'stuff' (the dark?).