No they don't. NOBODY says that light slowed down by billions of times six thousand years ago.
Nobody- but you.
Even pseudo-scientists like Setterfield would not say such a ridiculous thing.
Setterfield and other Christian physicists indeed believe light has slowed down billions of times in just the last 6000 years.
OK Luke, let me see if I can explain this to you. Whenever you see dating of rocks, ancient fossils, etc... that is usually using a form of radiometric or radioactive dating based on radioactive decay. Now if light were many millions or billions of times faster just a few thousand solar years ago, rocks and fossils will appear to be billions of years old if we use today's rate of decay. See, decay would also be much faster.
So, these scientists are still going to say the universe is 14-15 billion years old, when this decay can be accounted for in a much shorter period of time.
Actually, these scientists know and understand this just as well as Setterfield, and that is why there is great opposition to this theory.
Now, there are galaxies that really are billions of light years away at today's speed of light. Science has argued the Earth must be very old for the starlight to be here now, but if light was billions of times faster in the recent past, then starlight would appear almost everywhere in the known universe instantly.
Now, you can go ahead and show everybody how ignorant you are and make more stupid remarks that proves you have no idea what you are talking about.