I want to point out that there are currently more than 450 mental disorders - this is a massive amount! I have included an excerpt below from the Wikipedia page which sets out this controversy and provides a current list for you to peruse
This is a collection of conditions that have childhood developmental issues as the common characteristic. Most of these conditions have symptoms that overlap with ADHD, thus causing confusion in diagnosis and therefore in treatment as well.
The most well known of these is Autism. Autism is defined as “a neurodevelopmental disorder characterised by impaired social interaction, verbal and non-verbal communication, and restricted and repetitive behaviour.”
Autism is also referred to as Autism Spectrum Disorder which simply means that people with autism can display a range of symptoms
Up until the publication of the DSM V in 2013, no one was “permitted” to have ADHD and Autism together - you could only one or the other.
The DSM V removed that exclusion, as it was found that successful treatments for both conditions overlapped.