You are right, but it can be argued that during the time the painting was made, vertical writing was the predominant form, and I don't know whether horizontal writing was a thing at the time in Japan...

That said, as I implied in my other reply, the whole idea is a bit silly...

Japanese is currently read and written from left to right. However, until about 80 years ago (before World War II), it was read and written from right to left—though this applied only to horizontal writing. Vertical writing is read from right to left, and this convention continues today; for example, Japanese comics (manga) are still read from right to left.