The above study was a literature search that looked at a total of four out of 1653 records. It does not reflect when the study began only that it ended in 12/2016.
Art therapy is akin to journaling where there is a bond between the writer and the page. Journaling is effective and requires only the ability to transfer thoughts and feelings into a diary or sheet of paper. The idea of art therapy would have the same connection with the paper or canvas. Few people have the required skills to create a "work of art" at any level. I can see where those with artistic skills would be able to make the connection with the medium they are most comfortable with. If someone who has had a long interest in creating art was able to make the connection with the medium of chalk, oils, water, or acrylics and apply it to paper or canvas, I can see it working for that person. That brings up the observation of a particular therapy "working for who it works for". The key in deciding if a therapeutic approach is working or not means looking at the patient's progress towards recovery, and evaluating not so much the "work of art" but the patients progress through the art.