I think you're proving my point here.
Let me pick out a couple points here:
1. The fact that no Democrat "self-describes" as an athiest.
2. The fact that "If I run into a white person who describes themselves as Christian within the first ten minutes, I have a pretty good idea they are..."
and pose a simple narrative.
Many athiests and nominal Christians do not like Christians in general, jump to quick conclusions about them, stop thinking once they assign someone to a particular box, and do not like the fact that they feel uncomfortable self-describing as athiests. In general, these folks tend to the Democratic party.
And despite the big tent rhetoric of saying there are no "self-described athiests", there's an undercurrent of anger (It's appalling. I can't be me while you're being Johnny Bible Thumper over there) and fear (I have to pretend to be something I'm not, I can't describe myself the way I really would).
Actual, serious-minded Christians tend to know this, and therefore don't consider the Democrats any sort of option at all. Very few rational people want to belong to a group that they feel doesn't like or respect them.