I wonder how much of that might be due to under-diagnosing. Perhaps people presenting with ASD traits are more likely to be seen by a psychologist, and therefore more likely to get diagnosed with ADHD at the same time? (of course, it could also be the reverse of this)

I wasn't diagnosed with ADHD until I was a junior in college. I kinda knew I had it for some time before that (my ACT score vs my GPA was a good starting point...), but managed to survive without feeling the need to get treatment. Interestingly, I do have a sister and two nephews on the autism spectrum, so perhaps there are common genetics at play.