Frankly, I'm confused by your confusion.

The preceding statement for that list of definitions clearly states, "This is the spectrum of color commonly referred to as a 'color wheel.' We organize the spectrum on the color wheel by primary, secondary, and tertiary colors."

In the context of that statement and those definitions for what is COMMONLY referred to as the "color wheel," there is no cause for confusion.

It's basic theory every grade school child learns.