Overdiagnosed, maybe, definitely not fictitious. I got diagnosed at 26, and it took me a year of regular psychiatric visits (not psychologist, psychiatrist, the one that's an actual doctor), meeting at least once a month, before he was prepared to give me an official diagnosis. If you take your kid to a psychologist and they declare them ADHD after a session or two, then that's a problem. It needs to be established that it started really early (part of my evaluation involved digging out my Elementary school report cards; every single one back to Kindergarten had some version of "has trouble paying attention" on it), and that it's continuous, you don't have a "spell" of ADHD for a month and then go back to "normal" (you may be more or less impacted by it, but it's always there).
Also, kids REALLY shouldn't be rushed into being on a drug regimen. If you get offered drugs right off, find someone else to work with. Behavioural therapy can work for some cases, and there may be non-prescription options; I manage mine with caffeine. 80-100mg every 3-4 hours, as needed. I tried adderall and wellbutrin, but neither was as effective as caffeine and had side effects I didn't like.