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.
Therapy to deal with childhood trauma can absolutely benefit people with ADHD. Having ADHD doesn't magically make you immune to PTSD or other impacts of childhood trauma. It just means you also have ADHD. I know this because I have both ADHD and C-PTSD, and the ADHD was very, very present by the time the trauma started. Trauma symptoms exacerbate ADHD symptoms, and often vice versa. Both need to be treated. If kids' ADHD symptoms improve with trauma treatment, that doesn't necessarily mean they never had ADHD. It just means their overlapping symptoms might be less difficult to handle or, just as likely, mask. Ignoring the reality of overlap between trauma and ADHD serves only to erase people who have both. It benefits no one.
I'd go further and claim anyone benefits from trauma therapy, which is basically about healthy boundaries between you and the world, how nobody but you yourself are responsible for your feelings, how to regulate and enjoy every emotion, how to accept and welcome death, nonviolent communication, trauma-sensitive awareness of your own thoughts and (re)actions, how to tune your own belief systems more towards your own enjoyment and the benefit of those around you, the power of stories you tell yourself about the world and the power of rituals, etc.
No, I definitely didn't mean to say a kid with ADHD can't also have C-PTSD.
What I meant was that a kid who doesn't have any background of trauma can still have ADHD, but does not have PTSD, and no amount of therapy for childhood trauma will help their ADHD symptoms. That's my point - that there is overlap in symptoms but they are still different problems, that require different treatment.
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.