Not just a side effect; isn't this why stimulants like Ritalin and Adderall are used as treatment for hyperactivity and attention deficit?

Not exactly. Being hyper-focused on one thing is actually symptomatic of ADHD, and amphetamines like Adderall and Ritalin may exacerbate or mitigate this tendency (as I understand it mainly from personal experience, but also some literature), depending on the individual and external factors.

No, I'm pretty sure being "hyper-focused" is the opposite of ADHD, considering that "focus" and "attention" are synonyms and "deficit" and "hyper" are opposites,

And the effect of amphetamines is improved focus (as well as slight euphoria and wakefulness) BUT one possible problem is that people lose control of what they focus on and tend to get lost in the details. I. e. you start out writing a letter and end up reading about 18th century calligraphy.

> No, I'm pretty sure being "hyper-focused" is the opposite of ADHD

This is a common misconception: https://en.wikipedia.org/wiki/Hyperfocus

"It is typical for individuals with ADHD to say they 1), cannot focus on boring things and 2), can only focus on stimulating things, and that focus is often extreme. Thus it is both a concentration deficit and over-concentration, or generically: "hyperfocus." More concisely, some types of ADHD are a difficulty in directing one's attention, not a lack of attention"