If he believes the the Cambrian explosion is due to God then he believes in the fact of the event, but has a different causal agent (vs don't know)
He uses the very same evidence of the existence of the explosion and the absence of evidence to explain the explosion part of the evidence as reason to believe that "it is God". I usually end the dinner conversation when I keep asking "how?". On some very rare but weaker moments he might trip and admit "I don't know". At which point I end it by saying, that's a very scientific thing to say. :-)
He adds colour to our lives.
I think the axioms you need for this kind of argument is that: is God exists, God is human-like, and God loves us.
Why would a cosmic being care more about humans than bacteria? Why would he value things like freedom over slavery? Why would he care if we believed in him or not?
Those are entirely human qualities and concepts.
These are entirely human concepts only if you've already accepted some different axioms of your own. The Christian argument is that valuing freedom over slavery, and wanting worship, are divine qualities and concepts. And humans share those things because they were made after God's likeness for the sake of being loved (and more importantly, loving God in return).
There is no reason to think that if there is some sort of higher power, it has an any bigger interest in humans than it has with atoms or planets. To assume that is an assumption, which shouldn't be hidden.
If this is false, then I think some further axioms may be needed to differentiate the two scenarios.
I'm always perfectly happy when someone calls the big bang the moment of creation. It could have been.
Not really true.
"We don't know exactly how the Cambrian explosion came about, but have a lot of good evidence on how evolution works, which allows us to probe deeper into theories about what happened" is not equivalent to "it's all random chance!"
I do not differentiate between agnostic and atheistic thought. They are both the same thinking/belief systems in my view. An agnostic believes there could be a "god/creator" (but does not pursue this thought any further than "maybe"). An agnostic also believes there may not be a God/Creator given the absence of evidence to justify such a belief. An atheist believes that there is NO god/creator given the absence of evidence to prove the existence of such a being. If evidence had to exist then the atheist will believe the existence of such a being based on such evidence. That sounds exactly the same.
As to your point about "we don't know" - well, not knowing is perfectly fine in science. We cannot ascribe to or embellish and fabricate imagined facts and agents to any unknown phenomenon without evidence.
I find it very interesting that in de US there seem to be a lot of gnostic atheists (they know for certain there is no god, and are very vocal about this), while in Europe most people would be agnostic atheists, and they don't care as much.
My impression is Biblical literalism is more widespread in the US compared to Europe, which in turn is easier to decide for or against. Especially the discussion about evolution vs religion is almost entirely absent in Europe. Framing evolution as opposed to a belief in God means it is pretty easy to become an atheist, since we have overwhelming evidence for evolution.