Raw Histogram??

The statement that the lightest element in the scene has a reflectance of 100% seems to ignore specularity.

It is an assumption made for the purposes of the following text in the link.

Beginners please note that "the statement" does not apply to all scenes in the Real World

Or even most real world scenes.

Per Jones & Condit, 'brightness scale' ('dynamic range') of typical outdoor scenes varies from 4.75 EV to 9.57 EV.

The Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure
LOYD A. JONES AND H. R. CONDIT
Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York

Covered here

Beginners please note that owning and using a Kodak 18% gray card in the manner referred to by @IliahBorg would be almost unheard of these days, although the principles mentioned by Kerr remain true to this day.

I know ;)

Does it relate to this?

www.aghost.net/images/e0186601/ahistorylessonofrailroadtracks.pdf

It is most definitely incorrect, technically, to conflate "relatively ISO-invariant" with "totally-ISO-irrelevant" or "ISO-less", but there are differences in opinion, based on individual experience, that cause people to draw the line at different points in the sand as to whether the difference is practically relevant. Jim and I probably see the underlying empirical facts in a similar way, but we have very different editorial comments on practice, resulting from our own different subject matter.

On the opposite extreme is a camera like the Nikon D5, which still has some of the lowest pre-gain read noise in the industry, but has about the worst post-gain read noise in the industry, with currently-produced cameras. The newer on-sensor column-ADC designs are somewhere in-between.

And probably different opinions on what are acceptable amounts of image noise. Almost all the time, I call the image too noisy before the read noise begins to play a role in the visible noise.

Yes, because Kodak was once arguably a "bureaucracy" in the World of Photography, probably long before I.S.O.

Indeed:

blog.kasson.com/the-last-word/nikon-d5-read-noise-vs-iso-setting/

And then there's QWERTY.

"Irrationally implemented" and "fake" are two different things, IMO.

The rule of forum efficiency goes something like this: make your posts 15% shorter by dropping relevant qualifying terms or conditionals to avoid seeming too meticulous, and suffer 5x as many posts from the resulting confusion.

... and to never capitalize stuff like "imo" because it makes sentences wider and less abstruse ..

... (distant whine) ... "but im on my smartphone" 😋

G.W. Whistler used 5', that was 1524 mm, establishing a new bureaucracy ;)