We will have to agree to disagree.Fluid across society as a whole, but not within most individuals.
Imo most self-identified atheists have an idea of what they think a god is. What this is or whether it is logically consistent is unimportant.
In real life, I've never met a single person who has claimed not to understand what a god is, it is a minority view. As such it doesn't have a big effect when making a generalisation about most atheists.
For what it is worth, I do not understand what a god is.
Sure, I make a lot of wild guesses and I try real hard to build a working model of what all those people are talking about. I also have a very strong suspicion that it is a futile and pointless effort.
None of that changes the fact that I do not understand what a god is.