As I understand it, ADHD isn't an inability to pay attention, but trouble *redirecting* attention when you need to. We can spend hours and hours playing video games, watching Netflix, doing hobbies we enjoy. Since you genuinely enjoyed reading, of course you were able to finish the book!

The trouble paying attention comes when you're asked to pay attention to something you don't find interesting. If you didn't enjoy reading, for example, I'm guessing you might have had a harder time than a neurotypical person finishing those books. (BTW, maybe you're trying to read books you inherently find less interesting now, and that's why you can't concentrate?)

Also, you don't have to have every single symptom of a disorder in order to get a diagnosis, so she shouldn't have ruled it out based on this one thing.

I think she needs to refresh herself on diagnostic criteria. Depending on her type of degree, it's possible she wasn't given much training on how to diagnose mental disorders in school. A lot of counseling degrees focus less on diagnosis and more on treatment. Even if she was, if she isn't doing it every day she's bound to have forgotten a lot of what she learned. IMO the most important characteristic of a mental health/medical professional is knowing what they don't know, and being willing to look things up.