Their "explanation" is that light has not/must not have traveled at a constant speed throughout history. So, determining dates of celestial bodies via the length of time it takes light to reach Earth is unreliable.
Plus, it contradicts the Bible. Which, of course, it does not.
Nothing so complicated. If you assume an all powerful and all knowing creator than that creator could create a universe
where the light and gravity waves and everything else is already 6000 light years from the earth traveling along a trajectory toward earth at the moment of creation. Try and prove that one false.