His position is that investigating the practical causes of depression and anxiety should be the first approach to diagnosis, not that it should be the only approach. It's not that cases of depression without another cause don't exist; he's just saying those cases are less common.
To compare it with physical pain, saying that most cases are symptoms of something else doesn't mean that the pain shouldn't be treated, nor that cases of unknown or genetic origin would be any less legitimate.
https://www.psychologytoday.com/blog/theory-knowledge/201603...
His position is that investigating the practical causes of depression and anxiety should be the first approach to diagnosis, not that it should be the only approach. It's not that cases of depression without another cause don't exist; he's just saying those cases are less common.
To compare it with physical pain, saying that most cases are symptoms of something else doesn't mean that the pain shouldn't be treated, nor that cases of unknown or genetic origin would be any less legitimate.