If you read a lot about it, for instance, you'd know that the hyperactivity element of ADHD is age-inappropriate hyperactivity, not hyperactivity full stop. It is normal for kids to be lively, but a red flag for the possibility of a serious issue is hyperactivity that's out of synch with the child's age.
And also, a disorder of internal self control can be compensated for to some extent by an environment that gives lots of external control.
Most people can get homework done with harsh immediate consequences for failure, people with ADHD will struggle without those harsh immediate consequences because their mechanisms for internal self control (maintaining focus, working memory, planning, attention) are weaker. That doesn't mean that those external consequences have actually solved the problem and shown the ADHD doesn't exist, all they've done is compensate for it a bit.
Finally... untreated ADHD doesn't leave a child or adult with no problems, it just causes more problems such as depression and anxiety, substance abuse problems... all sorts. Drug companies (and, in the US, doctors) can make plenty of money out of those too...