The Equatorial Axis is the intersection of a sphere’s surface with the plane perpendicular to the sphere’s axis of rotation and midway between the poles.
You are describing a circle (intersection of the surface of a sphere with a plane), whereas the pictorial representation is not a circle, but a straight line intersecting the surface of the planet at two points: 9˚ south latitude, 91˚ east longitude and 9˚ north latitude, 89˚ west longitude. Since an "axis" is generally a line, and your explanation is at odds with the diagram provided by the authors, I suggest your post fails to explain the significance of this odd "axis".
Huh? Isn’t that the “equator”?
“The Equatorial Axis is the intersection of a sphere’s surface with the plane perpendicular to the sphere’s axis of rotation and midway between the poles.”
What you are describing is the Equator.
My question was rhetorical.