That statement is far too simplistic.

18% reflectivity is middle gray (neutral/average) in a print... approximately 12.5% reflectivity is average for "a typical scene."

To properly use an 18% gray card you prop it up against something at 45˚, and the angle of the card reduces the light reflected back to the camera to the correct level (12.7%). If the card is (incorrectly) held perpendicularly then the exposure would need to be increased approximately 1/2 stop over metered.

But these days only the most simplistic of evaluative metering systems only average the scene's reflectivity... for nearly 20 years now Nikon's evaluative (matrix) metering has used a database of over 30k images to compare the scene against in order to help ensure correct exposure. And many modern cameras utilize a highlight bias (backlighting awareness), and even more advanced levels of AI scene/subject recognition in determining the correct exposure (among other things).

And none of this is entirely standardized. There are three different "standards" for determining the correct exposure and they will give different results when used; regardless of the metering mode/scene. The two most common are SOS (standard output saturation, primarily used by compact cameras) and REI (recommended exposure index, primarily used by DSLR's and larger formats)... with digital cameras REI generally results in an exposure around 1/2 stop less than a standard saturation measurement does; in order to help protect the harsh highlight clipping characteristic of digital sensors (which effectively results in 18% vs 12.5%).

For example, DXO measures output saturation to determine the "true ISO" against a camera's reported ISO (Nikon Z9's REI ISO in this case).