> "The 50mm lens is called a “normal” or “standard” lens because the way it renders perspective closely matches that of the human eye."
This sentence is confusing and wrong.
A perspective that "closely matches that of the human eye" has nothing to do with the focal distance of a lens.
Such a perspective is obtained whenever you look at a photograph from such a distance so that you will see it under the same angle under which it has been seen by the photo camera.
For a normal 50 mm lens, that means that you must look at a 4:3 photograph from a distance about twice the height of the photograph, e.g. from about 60 cm when looking at a photograph whose height is 30 cm and whose width is 40 cm.
The normal focal length of around 50 mm has been chosen after some experiments about which is the maximum vision angle under which a painting or photograph can be seen, when looking at the complete ensemble, and not at details, while being able to perceive correctly the perspective relationships inside the image. The conclusion was that the aspect ratio must be around 4:3 and the viewing distance about twice the image height.
So it is a distance that feels comfortable for humans when looking at the entire image. Being much farther away diminishes the perception of small details, while being much closer makes difficult the perception of the complete image simultaneously.
When a photograph is taken with a wide-angular lens, one would have to look at the image from a too small distance, to match the original perspective. When a photograph is taken with a long-focus lens, one would have to look at the photograph from too far away, to match the original perspective.
N.B. The correct viewing distance for the normal perspective is from about twice the height of the image. I have no idea who has originated the very widespread myth that the right distance is the diagonal of the image. A computation for either 4:3 or 3:2 images would show that their diagonals are much less than 50 mm, when viewed from the corresponding angle of a normal lens (e.g. the diagonal of a 36 mm x 24 mm image is 43 mm). Moreover, 50 mm is at the lower limit of the normal focal lengths. Many normal camera lenses had slightly longer focal lengths, up to 54 mm, or even 56 mm, which were even farther from the diagonal lengths of the images.