"The definition of atheist as not a theist is supported by numerous dictionaries"
Is it? Can you provide numerous examples? Clearly an atheist is not a theist but that is certainly not what the word itself means.
It would be tiresome to cut and paste a lot of examples. Most dictionaries I checked list disbelief in god (or a close variation) as one of the definitions. If you define theist as belief in god then the two terms would be closely related. We just have to define disbelief. It seems we come back to the same basic question. Is disbelief belief in the opposite or a mere reluctance to believe? Once again definitions vary but many express the idea of reluctance or uncertainty without requiring belief in the opposite. Therefore, an atheist would be just someone who doesn't meet the definition of a theist.
There are problems with many dictionary definitions. Most people look up a word to find what it means, rather than what it doesn't mean. However, atheist does literally mean not a theist (since the prefix a means not). It's often hard to come up with a positive definition that includes everything excluded by another definition, and some dictionaries don't seem to be trying very hard. Furthermore, most dictionary writers are probably theists (just an assumption based on probabilities). Also the origins of the term atheist have been used exclusively as an insult for centuries before anyone willingly applied the label to themselves, based on what it literally means. Words change over time. It would be unusual to continue to apply the term atheist as an insult to Christians. In a similar way, there is no need to continue to apply the term as in insult to those who don't view themselves as theists. (I view the more narrow definition of a theism as an active belief in there being no god as an insult to other atheists because someone who met that definition (in my opinion) would be either intellectually dishonest or mentally unstable or both.)