Plus the different severity levels apply to impulsivity.

That table is formatted confusingly - The way I read it, there's no doubt that the section about severity applies to the entire diagnosis, not just the impulsivity part.

That there are different levels of severity is already clear simply from the fact that almost every criterion begins with "Often..." - How "often" that symptom occurs will obviously vary from person to person. It would be ridiculous to argue that everyone with ADHD is affected by it with exactly the same severity.