I will just throw this out there because it is not my area of expertise but......... A few years ago I was involved with martial arts and part of the syllabus was a study of the Chinese Taoist philosophy. The basis of this was an ancient text called the "Tao Te Ching" and various translations are still available.
Our Shihan repeatedly quoted the opening lines in Chinese because they are the key to the whole thing and they became familiar to me so, one day I quoted them to a Chinese customer of mine. He (very politely) said that it made no sense to him at all. Apparently, comparing modern Chinese with the ancient written or spoken language is like comparing Shakespearean English with the modern version.
So, it is quite possible that the text on this painting is correctly rendered in the "old" form and equally possible that it was just copied from who knows where to make the picture look more oriental.