Webster's intent was to distinguish the strict from the unstrict or vague.
Jesus' declaration as one of His principles was "either you are with Me or against Me." Those who are sometimes with, automatically fall into the category of being against. This probabily includes the entirety of Christianity or close to it.
The only thing distinguishing those who are strictly adhering is their understanding of the principles. Thus it is the understanding that divides the fundamentalist from the non-fundamentalist even though that was not Websters thought at the time. It is still the consequence.
The atheist only has the statement, "If you call it "god" (whatever that might mean) then I don't believe in it."
This would be saying that if the word "god" came to mean an automobile, then an atheist is only someone who doesn't believe in automobiles.
The atheist is not trying to say that he doesn't believe in the word, yet as a whole, THEY have expressed no other uniform understanding. This means that some must understand (and thus strictly adhere) what a "god" is so as to not believe in it, where others have a different understanding of what a god is. This divides them.
The result is that some will say, "I don't believe in god", but when you look at what they DO believe in, you will find that thing that was being referred to as "god" is included. This means that there are going to be atheists who do not strictly adhere simply because they had no undersanding of what a god was.
Thus you have fundamentalist, and non-fundamentalist within the same group even though they only make one statement.
The fundamentalist strictly adheres only to what he understands to be the intent. When that intent is not specified as any more than a word void of definition, then how can there be any strictness other than avoiding the word itself?
Is that what you are saying an atheist really is, just someone who doesn't believe in the word "god"?
If not, then you can only distinguish a fundamentalist from a non-fundamentalist by the degree of understanding they carry. The atheist has no unified, defined understanding of what "god" means.