A purist is someone who wants something to be pure, meaning clean, without other ingredients, unaltered, nothing else than that one thing.
Every time I think of a pure watercolorist who uses nothing but watercolors, that is no gouache for white accents and no glaze to protect the image. So the tough guys.
Dogmatism or not, I can discover an interesting concept. The watercolor purists finally settle on an unification of the materials and make their works more distinguishable. The only (difference beside the quality of the colors that is used) is the painting process. This unification of common processes forms a basis for discussion and evaluation of individual works. It’s like a sport that takes place under certain rules and gives context for everyone’s accomplishments.
I think it’s a good idea to clarify the common ground to have a helpful conversation about art (or something else).