The CMBR is the spectrum of a nearly perfect black body, a
black body being something that absorbs all radiation falling on it (so that anything it
emits is completely determined by its temperature). So if the CMBR is God light, then God is a black body.
Besides, the relationship is quite obviously a pure accident when you look at the "let there be light" phrase in context.
At the time, after all, it says that "the Spirit of God was hovering over the waters" immediately prior to this statement. However, at and before the time the CMB was emitted, our universe was
exceedingly smooth, the same in all directions. There were no up or down, no way to be above anything, and no waters to speak of.
Then, if that weren't enough, the very next verse after "let there be light" removes all doubt, as he says, "God saw that the light was good, and he separated light from darkness." However, the CMB radiation has never been separated between light and dark: it's the same temperature, the same brightness everywhere.
It is only if you pick a single verse out of context that it seems to even remotely match reality. Instead, if you read the whole of Genesis 1 (plus verses 1-3 of Genesis 2), it is quite clear that this is a creation myth that is reasonably descriptive and completely divorced from reality. It is the exact sort of thing you would expect a group of ignorant tribesmen to come up with thousands of years ago. And though there are some statements which, if taken out of context, can be construed to sorta kinda fit reality, there isn't one single insight into the true nature of reality to be had. And it would be very easy to do this: a simple statement like "the Sun is a star" would have been an amazing insight that nobody had at the time.