Does high IQ give cause to doubt an ADHD diagnosis?
Sounds to me like someone is making a logical fallacy here, though the origin of this fallacy isn't clear to me.

We cannot go from 'poor academic performance', to 'not amounting to anything', to 'having a low IQ'. These are not relationships of cause and effect.

The motivation to do something ('amount to something', if you will) is driven primarily by the reward-center. The neurotransmitter 'dopamine' is considered to be a large player in this game. Dopamine tends to project to the areas associated with reward (and these areas are clustered into the abstraction known as the pleasure center). Incidentally, ADHD has shown to be a dysfunction of executive reasoning as a result of low dopamine levels.

IQ is not determined by executive functioning. IQ is almost independent of executive functioning. If they were interchangeable, then Albert Einstein would have a very low IQ compared to, say, Donald Trump. I say that it is 'almost' independent because it is not clear that there is no relationship between the two at all. Studies have shown that ADHD sufferers show an average increase of 4.5 IQ points after long-term use of dopamine-reuptake inhibitors. [1]

You might also be interested in this page to answer the second part(?) of your question. It is well-written and extremely insightful.



Sources:

[1] Millichap JG (2010). "Chapter 3: Medications for ADHD". In Millichap JG. Attention Deficit Hyperactivity Disorder Handbook: A Physician's Guide to ADHD (2nd ed.). New York: Springer. pp. 121–123.