I was using it in the sense that "God is an artist" and "mannerism" is his style.
El Greco was a mannerist. His style of elongated faces and figures was a "manneristic" view of his world. Some say he was astigmatic and hence , he saw the world due to his eye problem. Others say NO, he just liked to draw that way.
You missed the term in its seminal form as a "series of painterly schools" of form and color.
So , indeed the "a" was necessary in my use of the word