This seems like a myth. I have non-diabetic blood sugar but have a severe case of ADHD. Likewise some people eat sugar all day and have no symptoms. It’s true that consuming sugary foods worsens ADHD symptoms, but my parents fed me pretty healthy foods as a child when my symptoms were the worst. My favorite beverage is carbonated water with no sugar. I eat vegetables and protein. I’m pretty symptomatic.
If there’s a correlation, it’s indirect. Individuals with bad impulse control tend to overconsume sugar, moreso than overconsuming sugar causing the bad impulse control to begin with.