Although you are right in stating that the word "amateur" did not have in the early 20th century the negative connotation it has today, at no point in history - and certainly not at that time - were the words "amateur" and "artist" ever considered synonymous, as you state. Not in music, not in art, not in dance. In music, throughout the 19th century, the amateur musicians had a huge impact in the development of many genres (choral and chamber music mostly), but at no point were they considered the same as "artists".
Other problem is that, at the time you mention, very few people considered photography an art, at the same level as music, painting or dance. For most people, photography was synonym with documentary, and there is a strong possibility that 99% of the people who bought a Brownie never heard of Alfred Stieglitz. To associate the Brownie with amateurs and with art is nonsensical from a historical point of view.
So, not sure what point you're trying to make.