Most medical conditions are diagnosed entirely on symptoms, and then those symptoms are treated. Especially in psychiatry, where we understand so little about the brain- all conditions in the DSM are symptom based, not mechanistic.

We cannot have a scientific understanding of the underlying mechanisms behind a disease, when our biological understanding is way too primitive to allow that.

Medicine has been helping people effectively for thousands of years, long before science even existed, by treating symptoms. It is "scientific" in the sense that the symptoms for a specific disease are clearly defined, and the safety and effectiveness of a treatment is determined experimentally, for the group of people fitting those symptoms.

The concept of a neurodevelopmental disorder is also symptom based- there is a measured progression of certain abilities in the "average" person, and there is a measurable delay in those abilities with a neurodevelopmental disorder. Again, that is a symptom, and has nothing to do with understanding the mechanism.

Using a lack of mechanistic understanding to say a condition doesn't exist, or is meaningless is nonsense. It's a clearly defined set of symptoms that can make normal life extremely difficult for the people affected, and can be effectively treated. It has known causes- both environmental, and genetic, as well as known physical phenotypic traits that can be measured experimentally. All hints towards eventually increasing our understanding of the underlying biology.

I would argue we actually understand virtually nothing about essentially all medical conditions, even the most deadly and most treatable ones. By your same logic, you could say diabetes "does not exist" because we don't understand exactly why people stop producing or become resistant to insulin. Yet people without treatment die, and people with treatment can thrive, which is sufficient reason for having a disease category despite lack of mechanistic understanding.

It would be great if we understood the human body a lot better, but that will take a long time, and a lot more research. We shouldn't stop making peoples lives better in the short term.

As a researcher in the biomedical field for a long time, I have come to see mechanisms come and go for diseases over the years, yet the diseases themselves remain constant. Most of our historically popular mechanisms like "depression is low serotonin" turned out to be either misunderstandings, or gross oversimplifications. Yet that doesn't change the fact that certain treatments work for certain clusters of symptoms we define as a disease.

I think "scientism" in medicine has been very harmful, and is mostly a delusion, and often a type of fraud aimed at creating an illusion of credibility. Pretending that things have a deep mechanistic explanation or understanding when they do not, and then dismissing safe and effective treatments when the mechanism isn't understood, or dismissing traditional medicine from other cultures and time periods because it's "not scientific."

Indeed, defining medical conditions always involves a cultural context, and is therefore "arbitrary" in a sense. I could imagine, for example, that having an ADHD brain could actually be a huge benefit to a hunter gatherer, but harmful to a modern office worker. Many other cultures and time periods have concepts of disease conditions that are basically incomprehensible to us, because they represent human differences which are now mainstream and culturally acceptable, so, despite representing a real difference, they do not cause a problem for people affected in the context of our modern society, and are therefore not a disease to us.