Yes, they do imply different root causes. PTSD requires a root cause of experiencing trauma. ADHD is not PTSD because it is not caused by trauma, and therefore therapy to deal with childhood trauma will not benefit people with ADHD.
It is, however, very true that they can look the same. I work with kids who have been through trauma and it is only on the order of years since people started recognizing that in these kids, many ADHD symptoms were caused by trauma and could be reduced or go away entirely with trauma-specific therapy.