I don't think this is true about brain chemistry or that ADD is a life-long disability. Just speaking from experience here, I was diagnosed with ADD as an adult in my mid-20s and then started using IFS therapy (parts work) a couple years later for childhood trauma. After two years of IFS therapy my ADD is pretty much non-existent and my executive functioning is better than most people I know.
My hunch is that for a lot of people, ADD is a result of growing up in a home that felt unstable so they never gained a basic internal sense of stability that you need for things like task switching or staying focused.
You can't generalize your experience (trauma mimicking ADD symptoms) to 'ADD [is not] a life-long disability' -- people have symptoms for different reasons and there are biomarkers for ADD.
Which specific ADHD biomarkers are you referring to? There are some promising areas for research but I'm not aware that any are yet accepted for clinical purposes.
IMHO many people with ADHD diagnose has really some kind of PTSD that started when they were children. And I believe that kind of condition is treatable. It would need a long therapy of course and a lot of changes in life. And I say this as a person with ADHD diagnose.
(I'm not GP.) I don't think the question here is whether trauma is mimicking ADHD or ADHD is a result of trauma, either in general, or in any particular case.
I think people lean into the personality traits, 'neurotype', cognitive styles, talents, and inherent tendencies they have as kids when it comes to coping with trauma. You use the tools you have. If you have ADHD and a situation is unbearable, hyperfocus is a tool you have— or at least a pattern you can fall into— that can prevent you from being overwhelmed by the feelings that weigh on you when that situation is on your mind. It would be shocking to me if ADHDers didn't often end up relying on features of ADHD to adapt to traumatic situations.
These things are interrelated in people who have both, and I don't think it's generally easy (or necessary) to pick apart 'which is which' when it comes to a specific behavior or experience.
Internal Family Systems therapy is a form of therapy that uses the idea of different "parts" of ourselves, and postulates that we can heal these different parts of ourselves by accessing an inner source of compassion underneath all these "parts" - suppressed emotional pain, defense mechanisms, coping mechanisms, etc. I've found it to be the most effective form of therapy for healing anxiety, depression, and childhood trauma, when combined with other mind-body practices.
IFS is the "Internal Family Systems" model of psychotherapy. I'm coincidentally in the midst of reading No Bad Parts, which was written by the originator, and covers the basic ideas behind the practice and gives some instructions for exercises. I've enjoyed the book so far.
My hunch is that for a lot of people, ADD is a result of growing up in a home that felt unstable so they never gained a basic internal sense of stability that you need for things like task switching or staying focused.