As a person who has adult ADHD, its honestly not something that should be joked about. I have no idea if he really has it or not, but the stigma associated with ADD and ADHD is usually framed around laziness, unintelligent, or people that can not follow through with something, and this is honestly far from the truth. People who have ADD and ADHD think and react differently than others who do not have it, and those who have it have decreased abilities in some areas while also normally having abilities significant accelerated in other areas.
Here is an interesting article someone shared with me last night on the subject http://www.healthline.com/health/adhd/iq-adhd#2
Not a single medical journal to date includes lying and stealing as being a symptom of this "affliction". That may indeed be a good read for those that are interested in that but again; not a single medical journal to date includes lying and stealing as being a symptom of this "affliction".
I don't know where you've gotten this stigma about ADD or ADHD from. I've never associated or heard it associated with "laziness, unintelligent, or people that can not follow through with something." I've only heard it framed around a learning disability and that it's difficult for sufferers to pay attention for extended periods of time. However I believe the affliction to be misdiagnosed and over-diagnosed in children and young adults for the sole purpose of prescribing medication pushed by Pharmaceutical compoany reps. It's also a bit of pop-psychology as even 3 family members of mine that are all psychologist's agree. The fact is that many children and young adults don't pay attention in school. They don't because they are children and young adults and school is not that interesting to many at those ages. I understand there are those that truly suffer from it and I am in no way discounting that.
True adult ADHD really has not much to do with "being able to pay attention for extended periods of time". It deals more with being unable to read social and facial cues, detachment from emotional aspects in relationships, the ability to "cubby-hole" issues and not deal with issues, and the ability to hyper-fixate on single elements over extended periods of time to the detriment of other aspects of life. The response about "not able to pay attention" is more of the pop psychology that doesn't understand where true adult ADHD resides on the Autism spectrum.