I don't understand how the surface is "cylindrical" I don't see a cylinder in the attached image. I am wondering what "cylinder" and "surface" they are talking about when they claim "the cylinder cuts through this surface".
A cylinder is the cross product of a disk and some subset of $\mathbb{R}$. The image above that winds and is a wall is not the cross product of a disk and some subset of $\mathbb{R}$. A wall cannot be a cylinder!
In a general context, a “cylindrical surface” is a surface formed by the motion of a line (the generator) moving parallel to itself and intersecting a given curve (the directrix).
https://www.encyclopediaofmath.org/index.php/Cylindrical_surface_(cylinder)