I suppose it is because the text discusses the traditional primary colors of pigments, rather than light. Then again, the text brings this very concept of categorization into question immediately.
Green is mentioned a few times. As caseyavila points out, the article is about colours historically used when painting - aka 'subtractive colours' - in which green isn't a primary colour. As per TFA, you can mix a muddy green from blue and yellow or use a specialised green pigment, as they become available historically. In light-based systems (as in screen RGB), green is in contrast a primary colour. Notice that subtractive colour systems (print ink, pigments, etc) often employ a fourth 'key' colour to get true blacks.
On the subject of green as a colour in its own right, the human visual system excels at recognising different shades of green compared with red.