I'm sorry, but have you looked at the TOVA API[1]?
I have.
Put simply, it's bullshit — and one that doesn't even purport to measure ADHD (which, indeed, is not something one could measure on a linear scale).
It has nothing to do with how we experience ADHD symptoms and traits.
It does, however, look like something one could train for by... playing videogames.
Surprise, playing videogames makes one statistically better at videogames! Now pay me.
Disclaimer: I have ADHD, and have written a small wiki about it, which quite a few folks with ADHD have found useful:
https://romankogan.net/adhd
The same remains to be said of the product we're discussing.
[1] https://tovatest.com/how-tova-works/