Adams’ point is mainly that photographs record the light coming from the subject to the camera—that is, reflected light—not the raw illumination falling onto the subject.

For Adams’ work, reflected readings were often more useful than incident readings for two reasons:

1. Landscape practicality: Much of his subject matter was distant terrain. You often can’t walk into the scene to take a meaningful incident reading, and even if you could, the light may change before you return.
2. Creative control: Incident metering tells you the general illumination, but not how bright or dark different subjects will appear to the camera. Adams wanted to know the luminance of specific parts of a scene, which is why narrow-angle reflected spot metering fit his Zone System approach so well.

An incident reading can be similar in intent to a gray-card assumption: useful for average exposure, but less informative when you want to place dark rocks, bright snow, or clouds precisely on the tonal scale.

So Adams wasn’t necessarily saying built-in reflective meters were common; he was stressing that exposure decisions in photography are ultimately based on the light the camera actually receives from the scene.