“I wonder if he’s wrong, of course.”
He is, of course.
There are lots of reasons why this is not true. Being challenged and interested in what you are studying, as you indicated, is one. Also, people with ADHD in many cases have a higher than average IQ, which allows us to do well despite our faults. And just having the right teachers and a supportive environment can make a huge difference.
It is strange that you continued to do well when you got to college. Typically the lack of structure and increased freedom in college spells doom for the ADDer. Of course, that is not always the case,and ADDers are anything but typical. 😉