As the article says, not everyone can use a scroll wheel, and not everyone even has a scroll wheel or touchpad. I personally find using mouse scroll wheels quite painful due to RSI caused from decades of mouse abuse. Yet, designers find scroll bars and up-and-down scroll arrows icky so fuck me, I guess. :(
Obviously not, but you also shouldn't expect the whole world to be designed around your needs. If you have difficulty using the expected input method, you should use alternatives with the same functionality, not expect others to redesign their apps in your preferred way.
Often, you don't even need to lift a finger, design-wise or implementation-wise, to have good accessibility. The system-wide or toolkit defaults are usually quite accessible right out of the box. macOS's horrible disappearing scrollbars being a huge exception.
Most of the terrible designs and accessibility problems stem from software going out its way to write custom controls or force controls to look and behave in a non-default way. It's not an afterthought--people are deliberately adding code to make their software worse.
"Accessibility" here refers to one person's very particular disability. You can't possibly predict the entire gamut of disabilities people might face. Trying to do so "first and foremost" is utter madness.
Nobody's asking you to predict anything, they're asking you to leave the functional design elements of a scrollbar alone and not fuck them up. Probably if you rent an angle grinder and remove the handrails from the steps at your local library it'll look nicer, but you don't do that, because there's a good reason for those steps to have handrails.
Designing with accessibility in mind makes software better for everyone, period.
You design a clear, visible, distinctive scroll bar and buttons? Boom, you've catered to everyone with bad vision (for any reason: from blindness to eye surgery), reduced motor skills (for any reason: from arthritis to old age to hand injuries), non-technical users (so they don't have to hunt around the interface hoping to discover hidden features).
And this goes for everything in software.
Disability is a spectrum, and you yourself will be disabled in one way or another, multiple times, during the course of your life. Be it from old age, surgeries, injuries, strain from sport or mundane tasks like holding a baby.
There are a number of common disabilities. Everyone starts losing near vision just after 40, which works out to almost half of the people using a computer expected lifespan is around 78, and babies don't use computers). 10% of the population is color blind of some sort. Most people will have a broken arm sometime in their life (the only statistic I can find is 6 million people in the US break their arm every year: this could also be a few clumsy people breaking their arm several times per year and most don't ever. I think most people breaking their arm at some point seems more likely)
