Is scrolling considered a "keyboard input"?
Scrolling can be a keyboard input. For example, using the arrow keys on a keyboard will often scroll the window.

While I have no experience with the Scroll Reverser app, a quick check shows the source code is available. And, according to its Read Me file:


  Scroll Reverser installs an event tap, which gives access to event stream, including scrolling events and gesture events. The main documentation is Quartz Event Services Reference.
  
  To distinguish between trackpad and mouse, Scroll Reverser essentially
  looks at the gesture events to determine whether there are 2 or more
  fingers on the trackpad. If so, it assumes it is the trackpad.


Since the app uses Quartz Event Services then it's basically monitoring low level inputs from users. More technical information is available on the Quartz Event Services page on Apple's Developer site.

But essentially, monitoring low level inputs also means monitoring keyboard inputs.