Yeah, I totally get that sign language is a real language.
It's that I don't expect a dictionary to have the same level of precision as a medical dictionary or encyclopedia. It similarly talks about kidneys as a pair of organs on the right and left of the abdomen, and the liver as a large organ in the upper abdomen.
No mention that the kidneys are made of nephrons, their connection to the bladder or role regulating osmotic equilibrium.