That's irrelevant.
The speed of light in a vacuum, or c, is a constant; it doesn't change. The fact that light slows down when passing through matter does not change the value of c.
If the top speed of your car is 120, then it stays that way. Just because you slow down to 55 around a turn does not change what the top speed is.
What the creationists here are claiming, is that c might have been different in the past, not that light slows down when passing through matter.
“That’s irrelevant.”
Not to my statement I was defending it wasn’t. I said the speed of light is not a constant. It isn’t.