It is not an assumption as it is based on my experience. That makes it anecdotal and biased but not unwarranted and not an assumption.
Your generalizing all atheists based on personal experience declaring most, fallacy big time. Does your experience include most atheists?
The distinctions are more defined than you understand. I wouldn't even call atheism a position, it is just a statement about an inner state. Agnosticism is also a statement about an inner state "I don't know what a god is" but it is also a position "and neither do you". I.e. the atheist doesn't make a falsifiable, objective claim, the Agnostic does.
Atheists dor not claim to make a falsifiable, objective claim, except that there is no objective evidence to believe, which is true. The limits of Methodological Naturalism are neutral as ro whether God(s0 exist or not. Statements of philosophical beliefs are not objectively based.